Nicksxs's Blog

What hurts more, the pain of hard work or the pain of regret?

这次单独介绍下docker体系里非常重要的cgroup,docker对资源的限制也是基于cgroup构建的,
简单尝试
新建一个shell脚本

#!/bin/bash
while true;do
    echo "1"
done

直接执行的话就是单核100%的cpu

首先在cgroup下面建个目录

mkdir -p /sys/fs/cgroup/cpu/sxs_test/

查看目录下的文件

其中cpuacct开头的表示cpu相关的统计信息,
我们要配置cpu的额度,是在cpu.cfs_quota_us中

echo 2000 > /sys/fs/cgroup/cpu/sxs_test/cpu.cfs_quota_us  

这样表示可以使用2%的cpu,总的配额是在cpu.cfs_period_us中

然后将当前进程输入到cgroup.procs,

echo $$ > /sys/fs/cgroup/cpu/sxs_test/cgroup.procs

这样就会自动继承当前进程产生的新进程
再次执行就可以看到cpu被限制了

执行细节
首先设置了默认的languageDriver
org/mybatis/mybatis/3.5.11/mybatis-3.5.11-sources.jar!/org/apache/ibatis/session/Configuration.java:215
configuration的构造方法里

languageRegistry.setDefaultDriverClass(XMLLanguageDriver.class);

而在
org.apache.ibatis.builder.xml.XMLStatementBuilder#parseStatementNode
中,创建了sqlSource,这里就会根据前面的 LanguageDriver 的实现选择对应的 sqlSource

SqlSource sqlSource = langDriver.createSqlSource(configuration, context, parameterTypeClass);

createSqlSource 就会调用

@Override
public SqlSource createSqlSource(Configuration configuration, XNode script, Class<?> parameterType) {
  XMLScriptBuilder builder = new XMLScriptBuilder(configuration, script, parameterType);
  return builder.parseScriptNode();
}

再往下的逻辑在 parseScriptNode 中,org.apache.ibatis.scripting.xmltags.XMLScriptBuilder#parseScriptNode

public SqlSource parseScriptNode() {
  MixedSqlNode rootSqlNode = parseDynamicTags(context);
  SqlSource sqlSource;
  if (isDynamic) {
    sqlSource = new DynamicSqlSource(configuration, rootSqlNode);
  } else {
    sqlSource = new RawSqlSource(configuration, rootSqlNode, parameterType);
  }
  return sqlSource;
}

首先要解析dynamicTag,调用了org.apache.ibatis.scripting.xmltags.XMLScriptBuilder#parseDynamicTags

protected MixedSqlNode parseDynamicTags(XNode node) {
    List<SqlNode> contents = new ArrayList<>();
    NodeList children = node.getNode().getChildNodes();
    for (int i = 0; i < children.getLength(); i++) {
      XNode child = node.newXNode(children.item(i));
      if (child.getNode().getNodeType() == Node.CDATA_SECTION_NODE || child.getNode().getNodeType() == Node.TEXT_NODE) {
        String data = child.getStringBody("");
        TextSqlNode textSqlNode = new TextSqlNode(data);
        // ---------> 主要是这边的逻辑
        if (textSqlNode.isDynamic()) {
          contents.add(textSqlNode);
          isDynamic = true;
        } else {
          contents.add(new StaticTextSqlNode(data));
        }
      } else if (child.getNode().getNodeType() == Node.ELEMENT_NODE) { // issue #628
        String nodeName = child.getNode().getNodeName();
        NodeHandler handler = nodeHandlerMap.get(nodeName);
        if (handler == null) {
          throw new BuilderException("Unknown element <" + nodeName + "> in SQL statement.");
        }
        handler.handleNode(child, contents);
        isDynamic = true;
      }
    }
    return new MixedSqlNode(contents);
  }

判断是否是动态sql,调用了org.apache.ibatis.scripting.xmltags.TextSqlNode#isDynamic

public boolean isDynamic() {
  DynamicCheckerTokenParser checker = new DynamicCheckerTokenParser();
  // ----------> 主要是这里的方法
  GenericTokenParser parser = createParser(checker);
  parser.parse(text);
  return checker.isDynamic();
}

创建parser的时候可以看到这个parser是干了啥,其实就是找有没有${ , }

private GenericTokenParser createParser(TokenHandler handler) {
  return new GenericTokenParser("${", "}", handler);
}

如果是的话,就在上面把 isDynamic 设置为true 如果是true 的话就创建 DynamicSqlSource

sqlSource = new DynamicSqlSource(configuration, rootSqlNode);

如果不是的话就创建RawSqlSource

sqlSource = new RawSqlSource(configuration, rootSqlNode, parameterType);
```java

但是这不是一个真实可用的 `sqlSource` ,
实际创建的时候会走到这
```java
public RawSqlSource(Configuration configuration, SqlNode rootSqlNode, Class<?> parameterType) {
    this(configuration, getSql(configuration, rootSqlNode), parameterType);
  }

  public RawSqlSource(Configuration configuration, String sql, Class<?> parameterType) {
    SqlSourceBuilder sqlSourceParser = new SqlSourceBuilder(configuration);
    Class<?> clazz = parameterType == null ? Object.class : parameterType;
    sqlSource = sqlSourceParser.parse(sql, clazz, new HashMap<>());
  }

具体的sqlSource是通过org.apache.ibatis.builder.SqlSourceBuilder#parse 创建的
具体的代码逻辑是

public SqlSource parse(String originalSql, Class<?> parameterType, Map<String, Object> additionalParameters) {
  ParameterMappingTokenHandler handler = new ParameterMappingTokenHandler(configuration, parameterType, additionalParameters);
  GenericTokenParser parser = new GenericTokenParser("#{", "}", handler);
  String sql;
  if (configuration.isShrinkWhitespacesInSql()) {
    sql = parser.parse(removeExtraWhitespaces(originalSql));
  } else {
    sql = parser.parse(originalSql);
  }
  return new StaticSqlSource(configuration, sql, handler.getParameterMappings());
}

这里创建的其实是StaticSqlSource ,多带一句前面的parser是将原来这样select * from student where id = #{id}sql 解析成了select * from student where id = ? 然后创建了StaticSqlSource

public StaticSqlSource(Configuration configuration, String sql, List<ParameterMapping> parameterMappings) {
  this.sql = sql;
  this.parameterMappings = parameterMappings;
  this.configuration = configuration;
}

为什么前面要讲这么多好像没什么关系的代码呢,其实在最开始我们执行sql的代码中

@Override
  public <E> List<E> query(MappedStatement ms, Object parameterObject, RowBounds rowBounds, ResultHandler resultHandler) throws SQLException {
    BoundSql boundSql = ms.getBoundSql(parameterObject);
    CacheKey key = createCacheKey(ms, parameterObject, rowBounds, boundSql);
    return query(ms, parameterObject, rowBounds, resultHandler, key, boundSql);
  }

这里获取了BoundSql,而BoundSql是怎么来的呢,首先调用了org.apache.ibatis.mapping.MappedStatement#getBoundSql

public BoundSql getBoundSql(Object parameterObject) {
    BoundSql boundSql = sqlSource.getBoundSql(parameterObject);
    List<ParameterMapping> parameterMappings = boundSql.getParameterMappings();
    if (parameterMappings == null || parameterMappings.isEmpty()) {
      boundSql = new BoundSql(configuration, boundSql.getSql(), parameterMap.getParameterMappings(), parameterObject);
    }

    // check for nested result maps in parameter mappings (issue #30)
    for (ParameterMapping pm : boundSql.getParameterMappings()) {
      String rmId = pm.getResultMapId();
      if (rmId != null) {
        ResultMap rm = configuration.getResultMap(rmId);
        if (rm != null) {
          hasNestedResultMaps |= rm.hasNestedResultMaps();
        }
      }
    }

    return boundSql;
  }

而我们从上面的解析中可以看到这里的sqlSource是一层RawSqlSource , 它的getBoundSql又是调用内部的sqlSource的方法

@Override
public BoundSql getBoundSql(Object parameterObject) {
  return sqlSource.getBoundSql(parameterObject);
}

内部的sqlSource 就是StaticSqlSource

@Override
public BoundSql getBoundSql(Object parameterObject) {
  return new BoundSql(configuration, sql, parameterMappings, parameterObject);
}

这个BoundSql的内容也比较简单

public BoundSql(Configuration configuration, String sql, List<ParameterMapping> parameterMappings, Object parameterObject) {
  this.sql = sql;
  this.parameterMappings = parameterMappings;
  this.parameterObject = parameterObject;
  this.additionalParameters = new HashMap<>();
  this.metaParameters = configuration.newMetaObject(additionalParameters);
}

而上次在这边org.apache.ibatis.executor.SimpleExecutor#doQuery 的时候落了个东西,就是StatementHandler的逻辑

@Override
public <E> List<E> doQuery(MappedStatement ms, Object parameter, RowBounds rowBounds, ResultHandler resultHandler, BoundSql boundSql) throws SQLException {
  Statement stmt = null;
  try {
    Configuration configuration = ms.getConfiguration();
    StatementHandler handler = configuration.newStatementHandler(wrapper, ms, parameter, rowBounds, resultHandler, boundSql);
    stmt = prepareStatement(handler, ms.getStatementLog());
    return handler.query(stmt, resultHandler);
  } finally {
    closeStatement(stmt);
  }
}

它是通过statementType来区分应该使用哪个statementHandler,我们这使用的就是PreparedStatementHandler

public RoutingStatementHandler(Executor executor, MappedStatement ms, Object parameter, RowBounds rowBounds, ResultHandler resultHandler, BoundSql boundSql) {

  switch (ms.getStatementType()) {
    case STATEMENT:
      delegate = new SimpleStatementHandler(executor, ms, parameter, rowBounds, resultHandler, boundSql);
      break;
    case PREPARED:
      delegate = new PreparedStatementHandler(executor, ms, parameter, rowBounds, resultHandler, boundSql);
      break;
    case CALLABLE:
      delegate = new CallableStatementHandler(executor, ms, parameter, rowBounds, resultHandler, boundSql);
      break;
    default:
      throw new ExecutorException("Unknown statement type: " + ms.getStatementType());
  }

}

所以上次有个细节可以补充,这边的doQuery里面的handler.query 应该是调用了PreparedStatementHandler 的query方法

@Override
public <E> List<E> doQuery(MappedStatement ms, Object parameter, RowBounds rowBounds, ResultHandler resultHandler, BoundSql boundSql) throws SQLException {
  Statement stmt = null;
  try {
    Configuration configuration = ms.getConfiguration();
    StatementHandler handler = configuration.newStatementHandler(wrapper, ms, parameter, rowBounds, resultHandler, boundSql);
    stmt = prepareStatement(handler, ms.getStatementLog());
    return handler.query(stmt, resultHandler);
  } finally {
    closeStatement(stmt);
  }
}

因为上面prepareStatement中getConnection拿到connection是com.mysql.cj.jdbc.ConnectionImpl#ConnectionImpl(com.mysql.cj.conf.HostInfo)

@Override
public <E> List<E> query(Statement statement, ResultHandler resultHandler) throws SQLException {
  PreparedStatement ps = (PreparedStatement) statement;
  ps.execute();
  return resultSetHandler.handleResultSets(ps);
}

那又为什么是这个呢,可以在网上找,我们在mybatis-config.xml里配置的

<transactionManager type="JDBC"/>

因此在parseConfiguration中配置environment时

private void parseConfiguration(XNode root) {
    try {
      // issue #117 read properties first
      propertiesElement(root.evalNode("properties"));
      Properties settings = settingsAsProperties(root.evalNode("settings"));
      loadCustomVfs(settings);
      loadCustomLogImpl(settings);
      typeAliasesElement(root.evalNode("typeAliases"));
      pluginElement(root.evalNode("plugins"));
      objectFactoryElement(root.evalNode("objectFactory"));
      objectWrapperFactoryElement(root.evalNode("objectWrapperFactory"));
      reflectorFactoryElement(root.evalNode("reflectorFactory"));
      settingsElement(settings);
      // read it after objectFactory and objectWrapperFactory issue #631
      // ----------> 就是这里
      environmentsElement(root.evalNode("environments"));
      databaseIdProviderElement(root.evalNode("databaseIdProvider"));
      typeHandlerElement(root.evalNode("typeHandlers"));
      mapperElement(root.evalNode("mappers"));
    } catch (Exception e) {
      throw new BuilderException("Error parsing SQL Mapper Configuration. Cause: " + e, e);
    }
  }

调用的这个方法通过获取xml中的transactionManager 配置的类型,也就是JDBC

private void environmentsElement(XNode context) throws Exception {
  if (context != null) {
    if (environment == null) {
      environment = context.getStringAttribute("default");
    }
    for (XNode child : context.getChildren()) {
      String id = child.getStringAttribute("id");
      if (isSpecifiedEnvironment(id)) {
        // -------> 找到这里
        TransactionFactory txFactory = transactionManagerElement(child.evalNode("transactionManager"));
        DataSourceFactory dsFactory = dataSourceElement(child.evalNode("dataSource"));
        DataSource dataSource = dsFactory.getDataSource();
        Environment.Builder environmentBuilder = new Environment.Builder(id)
            .transactionFactory(txFactory)
            .dataSource(dataSource);
        configuration.setEnvironment(environmentBuilder.build());
        break;
      }
    }
  }
}

是通过以下方法获取的,

// 方法全限定名 org.apache.ibatis.builder.xml.XMLConfigBuilder#transactionManagerElement
private TransactionFactory transactionManagerElement(XNode context) throws Exception {
    if (context != null) {
      String type = context.getStringAttribute("type");
      Properties props = context.getChildrenAsProperties();
      TransactionFactory factory = (TransactionFactory) resolveClass(type).getDeclaredConstructor().newInstance();
      factory.setProperties(props);
      return factory;
    }
    throw new BuilderException("Environment declaration requires a TransactionFactory.");
  }

// 方法全限定名 org.apache.ibatis.builder.BaseBuilder#resolveClass
protected <T> Class<? extends T> resolveClass(String alias) {
    if (alias == null) {
      return null;
    }
    try {
      return resolveAlias(alias);
    } catch (Exception e) {
      throw new BuilderException("Error resolving class. Cause: " + e, e);
    }
  }

// 方法全限定名 org.apache.ibatis.builder.BaseBuilder#resolveAlias
  protected <T> Class<? extends T> resolveAlias(String alias) {
    return typeAliasRegistry.resolveAlias(alias);
  }
// 方法全限定名 org.apache.ibatis.type.TypeAliasRegistry#resolveAlias
  public <T> Class<T> resolveAlias(String string) {
    try {
      if (string == null) {
        return null;
      }
      // issue #748
      String key = string.toLowerCase(Locale.ENGLISH);
      Class<T> value;
      if (typeAliases.containsKey(key)) {
        value = (Class<T>) typeAliases.get(key);
      } else {
        value = (Class<T>) Resources.classForName(string);
      }
      return value;
    } catch (ClassNotFoundException e) {
      throw new TypeException("Could not resolve type alias '" + string + "'.  Cause: " + e, e);
    }
  }

而通过JDBC获取得是啥的,就是在Configuration的构造方法里写了的JdbcTransactionFactory

public Configuration() {
  typeAliasRegistry.registerAlias("JDBC", JdbcTransactionFactory.class);

所以我们在这

private SqlSession openSessionFromDataSource(ExecutorType execType, TransactionIsolationLevel level, boolean autoCommit) {
  Transaction tx = null;
  try {
    final Environment environment = configuration.getEnvironment();
    final TransactionFactory transactionFactory = getTransactionFactoryFromEnvironment(environment);

获得到的TransactionFactory 就是 JdbcTransactionFactory ,而后

tx = transactionFactory.newTransaction(environment.getDataSource(), level, autoCommit);
```java

创建的transaction就是JdbcTransaction 
```java
  @Override
  public Transaction newTransaction(DataSource ds, TransactionIsolationLevel level, boolean autoCommit) {
    return new JdbcTransaction(ds, level, autoCommit, skipSetAutoCommitOnClose);
  }

然后我们再会上去看代码getConnection ,

protected Connection getConnection(Log statementLog) throws SQLException {
  // -------> 这里的transaction就是JdbcTransaction
  Connection connection = transaction.getConnection();
  if (statementLog.isDebugEnabled()) {
    return ConnectionLogger.newInstance(connection, statementLog, queryStack);
  } else {
    return connection;
  }
}

即调用了

  @Override
  public Connection getConnection() throws SQLException {
    if (connection == null) {
      openConnection();
    }
    return connection;
  }

  protected void openConnection() throws SQLException {
    if (log.isDebugEnabled()) {
      log.debug("Opening JDBC Connection");
    }
    connection = dataSource.getConnection();
    if (level != null) {
      connection.setTransactionIsolation(level.getLevel());
    }
    setDesiredAutoCommit(autoCommit);
  }
  @Override
  public Connection getConnection() throws SQLException {
    return popConnection(dataSource.getUsername(), dataSource.getPassword()).getProxyConnection();
  }

private PooledConnection popConnection(String username, String password) throws SQLException {
    boolean countedWait = false;
    PooledConnection conn = null;
    long t = System.currentTimeMillis();
    int localBadConnectionCount = 0;

    while (conn == null) {
      lock.lock();
      try {
        if (!state.idleConnections.isEmpty()) {
          // Pool has available connection
          conn = state.idleConnections.remove(0);
          if (log.isDebugEnabled()) {
            log.debug("Checked out connection " + conn.getRealHashCode() + " from pool.");
          }
        } else {
          // Pool does not have available connection
          if (state.activeConnections.size() < poolMaximumActiveConnections) {
            // Can create new connection
            // ------------> 走到这里会创建PooledConnection,但是里面会先调用dataSource.getConnection()
            conn = new PooledConnection(dataSource.getConnection(), this);
            if (log.isDebugEnabled()) {
              log.debug("Created connection " + conn.getRealHashCode() + ".");
            }
          } else {
            // Cannot create new connection
            PooledConnection oldestActiveConnection = state.activeConnections.get(0);
            long longestCheckoutTime = oldestActiveConnection.getCheckoutTime();
            if (longestCheckoutTime > poolMaximumCheckoutTime) {
              // Can claim overdue connection
              state.claimedOverdueConnectionCount++;
              state.accumulatedCheckoutTimeOfOverdueConnections += longestCheckoutTime;
              state.accumulatedCheckoutTime += longestCheckoutTime;
              state.activeConnections.remove(oldestActiveConnection);
              if (!oldestActiveConnection.getRealConnection().getAutoCommit()) {
                try {
                  oldestActiveConnection.getRealConnection().rollback();
                } catch (SQLException e) {
                  /*
                     Just log a message for debug and continue to execute the following
                     statement like nothing happened.
                     Wrap the bad connection with a new PooledConnection, this will help
                     to not interrupt current executing thread and give current thread a
                     chance to join the next competition for another valid/good database
                     connection. At the end of this loop, bad {@link @conn} will be set as null.
                   */
                  log.debug("Bad connection. Could not roll back");
                }
              }
              conn = new PooledConnection(oldestActiveConnection.getRealConnection(), this);
              conn.setCreatedTimestamp(oldestActiveConnection.getCreatedTimestamp());
              conn.setLastUsedTimestamp(oldestActiveConnection.getLastUsedTimestamp());
              oldestActiveConnection.invalidate();
              if (log.isDebugEnabled()) {
                log.debug("Claimed overdue connection " + conn.getRealHashCode() + ".");
              }
            } else {
              // Must wait
              try {
                if (!countedWait) {
                  state.hadToWaitCount++;
                  countedWait = true;
                }
                if (log.isDebugEnabled()) {
                  log.debug("Waiting as long as " + poolTimeToWait + " milliseconds for connection.");
                }
                long wt = System.currentTimeMillis();
                condition.await(poolTimeToWait, TimeUnit.MILLISECONDS);
                state.accumulatedWaitTime += System.currentTimeMillis() - wt;
              } catch (InterruptedException e) {
                // set interrupt flag
                Thread.currentThread().interrupt();
                break;
              }
            }
          }
        }
        if (conn != null) {
          // ping to server and check the connection is valid or not
          if (conn.isValid()) {
            if (!conn.getRealConnection().getAutoCommit()) {
              conn.getRealConnection().rollback();
            }
            conn.setConnectionTypeCode(assembleConnectionTypeCode(dataSource.getUrl(), username, password));
            conn.setCheckoutTimestamp(System.currentTimeMillis());
            conn.setLastUsedTimestamp(System.currentTimeMillis());
            state.activeConnections.add(conn);
            state.requestCount++;
            state.accumulatedRequestTime += System.currentTimeMillis() - t;
          } else {
            if (log.isDebugEnabled()) {
              log.debug("A bad connection (" + conn.getRealHashCode() + ") was returned from the pool, getting another connection.");
            }
            state.badConnectionCount++;
            localBadConnectionCount++;
            conn = null;
            if (localBadConnectionCount > (poolMaximumIdleConnections + poolMaximumLocalBadConnectionTolerance)) {
              if (log.isDebugEnabled()) {
                log.debug("PooledDataSource: Could not get a good connection to the database.");
              }
              throw new SQLException("PooledDataSource: Could not get a good connection to the database.");
            }
          }
        }
      } finally {
        lock.unlock();
      }

    }

    if (conn == null) {
      if (log.isDebugEnabled()) {
        log.debug("PooledDataSource: Unknown severe error condition.  The connection pool returned a null connection.");
      }
      throw new SQLException("PooledDataSource: Unknown severe error condition.  The connection pool returned a null connection.");
    }

    return conn;
  }

其实就是调用的

// org.apache.ibatis.datasource.unpooled.UnpooledDataSource#getConnection()
  @Override
  public Connection getConnection() throws SQLException {
    return doGetConnection(username, password);
  }
```java

然后就是
```java
private Connection doGetConnection(String username, String password) throws SQLException {
    Properties props = new Properties();
    if (driverProperties != null) {
      props.putAll(driverProperties);
    }
    if (username != null) {
      props.setProperty("user", username);
    }
    if (password != null) {
      props.setProperty("password", password);
    }
    return doGetConnection(props);
  }

继续这个逻辑

  private Connection doGetConnection(Properties properties) throws SQLException {
    initializeDriver();
    Connection connection = DriverManager.getConnection(url, properties);
    configureConnection(connection);
    return connection;
  }
    @CallerSensitive
    public static Connection getConnection(String url,
        java.util.Properties info) throws SQLException {

        return (getConnection(url, info, Reflection.getCallerClass()));
    }
private static Connection getConnection(
        String url, java.util.Properties info, Class<?> caller) throws SQLException {
        /*
         * When callerCl is null, we should check the application's
         * (which is invoking this class indirectly)
         * classloader, so that the JDBC driver class outside rt.jar
         * can be loaded from here.
         */
        ClassLoader callerCL = caller != null ? caller.getClassLoader() : null;
        synchronized(DriverManager.class) {
            // synchronize loading of the correct classloader.
            if (callerCL == null) {
                callerCL = Thread.currentThread().getContextClassLoader();
            }
        }

        if(url == null) {
            throw new SQLException("The url cannot be null", "08001");
        }

        println("DriverManager.getConnection(\"" + url + "\")");

        // Walk through the loaded registeredDrivers attempting to make a connection.
        // Remember the first exception that gets raised so we can reraise it.
        SQLException reason = null;

        for(DriverInfo aDriver : registeredDrivers) {
            // If the caller does not have permission to load the driver then
            // skip it.
            if(isDriverAllowed(aDriver.driver, callerCL)) {
                try {
                    // ----------> driver[className=com.mysql.cj.jdbc.Driver@64030b91]
                    println("    trying " + aDriver.driver.getClass().getName());
                    Connection con = aDriver.driver.connect(url, info);
                    if (con != null) {
                        // Success!
                        println("getConnection returning " + aDriver.driver.getClass().getName());
                        return (con);
                    }
                } catch (SQLException ex) {
                    if (reason == null) {
                        reason = ex;
                    }
                }

            } else {
                println("    skipping: " + aDriver.getClass().getName());
            }

        }

        // if we got here nobody could connect.
        if (reason != null)    {
            println("getConnection failed: " + reason);
            throw reason;
        }

        println("getConnection: no suitable driver found for "+ url);
        throw new SQLException("No suitable driver found for "+ url, "08001");
    }

上面的driver就是driver[className=com.mysql.cj.jdbc.Driver@64030b91]

// com.mysql.cj.jdbc.NonRegisteringDriver#connect
public Connection connect(String url, Properties info) throws SQLException {
        try {
            try {
                if (!ConnectionUrl.acceptsUrl(url)) {
                    return null;
                } else {
                    ConnectionUrl conStr = ConnectionUrl.getConnectionUrlInstance(url, info);
                    switch (conStr.getType()) {
                        case SINGLE_CONNECTION:
                            return ConnectionImpl.getInstance(conStr.getMainHost());
                        case FAILOVER_CONNECTION:
                        case FAILOVER_DNS_SRV_CONNECTION:
                            return FailoverConnectionProxy.createProxyInstance(conStr);
                        case LOADBALANCE_CONNECTION:
                        case LOADBALANCE_DNS_SRV_CONNECTION:
                            return LoadBalancedConnectionProxy.createProxyInstance(conStr);
                        case REPLICATION_CONNECTION:
                        case REPLICATION_DNS_SRV_CONNECTION:
                            return ReplicationConnectionProxy.createProxyInstance(conStr);
                        default:
                            return null;
                    }
                }
            } catch (UnsupportedConnectionStringException var5) {
                return null;
            } catch (CJException var6) {
                throw (UnableToConnectException)ExceptionFactory.createException(UnableToConnectException.class, Messages.getString("NonRegisteringDriver.17", new Object[]{var6.toString()}), var6);
            }
        } catch (CJException var7) {
            throw SQLExceptionsMapping.translateException(var7);
        }
    }

这是个 SINGLE_CONNECTION ,所以调用的就是 return ConnectionImpl.getInstance(conStr.getMainHost());
然后在这里设置了代理类

public PooledConnection(Connection connection, PooledDataSource dataSource) {
    this.hashCode = connection.hashCode();
    this.realConnection = connection;
    this.dataSource = dataSource;
    this.createdTimestamp = System.currentTimeMillis();
    this.lastUsedTimestamp = System.currentTimeMillis();
    this.valid = true;
    this.proxyConnection = (Connection) Proxy.newProxyInstance(Connection.class.getClassLoader(), IFACES, this);
  }

结合这个

@Override
public Connection getConnection() throws SQLException {
  return popConnection(dataSource.getUsername(), dataSource.getPassword()).getProxyConnection();
}

所以最终的connection就是com.mysql.cj.jdbc.ConnectionImpl@358ab600

先补充两个点,
第一是前面我们说了
使用org.apache.ibatis.builder.xml.XMLConfigBuilder 创建了parser解析器,那么解析的结果是什么
看这个方法的返回值

public Configuration parse() {
  if (parsed) {
    throw new BuilderException("Each XMLConfigBuilder can only be used once.");
  }
  parsed = true;
  parseConfiguration(parser.evalNode("/configuration"));
  return configuration;
}

返回的是 org.apache.ibatis.session.Configuration , 而这个 Configuration 也是 mybatis 中特别重要的配置核心类,贴一下里面的成员变量,

public class Configuration {

  protected Environment environment;

  protected boolean safeRowBoundsEnabled;
  protected boolean safeResultHandlerEnabled = true;
  protected boolean mapUnderscoreToCamelCase;
  protected boolean aggressiveLazyLoading;
  protected boolean multipleResultSetsEnabled = true;
  protected boolean useGeneratedKeys;
  protected boolean useColumnLabel = true;
  protected boolean cacheEnabled = true;
  protected boolean callSettersOnNulls;
  protected boolean useActualParamName = true;
  protected boolean returnInstanceForEmptyRow;
  protected boolean shrinkWhitespacesInSql;
  protected boolean nullableOnForEach;
  protected boolean argNameBasedConstructorAutoMapping;

  protected String logPrefix;
  protected Class<? extends Log> logImpl;
  protected Class<? extends VFS> vfsImpl;
  protected Class<?> defaultSqlProviderType;
  protected LocalCacheScope localCacheScope = LocalCacheScope.SESSION;
  protected JdbcType jdbcTypeForNull = JdbcType.OTHER;
  protected Set<String> lazyLoadTriggerMethods = new HashSet<>(Arrays.asList("equals", "clone", "hashCode", "toString"));
  protected Integer defaultStatementTimeout;
  protected Integer defaultFetchSize;
  protected ResultSetType defaultResultSetType;
  protected ExecutorType defaultExecutorType = ExecutorType.SIMPLE;
  protected AutoMappingBehavior autoMappingBehavior = AutoMappingBehavior.PARTIAL;
  protected AutoMappingUnknownColumnBehavior autoMappingUnknownColumnBehavior = AutoMappingUnknownColumnBehavior.NONE;

  protected Properties variables = new Properties();
  protected ReflectorFactory reflectorFactory = new DefaultReflectorFactory();
  protected ObjectFactory objectFactory = new DefaultObjectFactory();
  protected ObjectWrapperFactory objectWrapperFactory = new DefaultObjectWrapperFactory();

  protected boolean lazyLoadingEnabled = false;
  protected ProxyFactory proxyFactory = new JavassistProxyFactory(); // #224 Using internal Javassist instead of OGNL

  protected String databaseId;
  /**
   * Configuration factory class.
   * Used to create Configuration for loading deserialized unread properties.
   *
   * @see <a href='https://github.com/mybatis/old-google-code-issues/issues/300'>Issue 300 (google code)</a>
   */
  protected Class<?> configurationFactory;

  protected final MapperRegistry mapperRegistry = new MapperRegistry(this);
  protected final InterceptorChain interceptorChain = new InterceptorChain();
  protected final TypeHandlerRegistry typeHandlerRegistry = new TypeHandlerRegistry(this);
  protected final TypeAliasRegistry typeAliasRegistry = new TypeAliasRegistry();
  protected final LanguageDriverRegistry languageRegistry = new LanguageDriverRegistry();

  protected final Map<String, MappedStatement> mappedStatements = new StrictMap<MappedStatement>("Mapped Statements collection")
      .conflictMessageProducer((savedValue, targetValue) ->
          ". please check " + savedValue.getResource() + " and " + targetValue.getResource());
  protected final Map<String, Cache> caches = new StrictMap<>("Caches collection");
  protected final Map<String, ResultMap> resultMaps = new StrictMap<>("Result Maps collection");
  protected final Map<String, ParameterMap> parameterMaps = new StrictMap<>("Parameter Maps collection");
  protected final Map<String, KeyGenerator> keyGenerators = new StrictMap<>("Key Generators collection");

  protected final Set<String> loadedResources = new HashSet<>();
  protected final Map<String, XNode> sqlFragments = new StrictMap<>("XML fragments parsed from previous mappers");

  protected final Collection<XMLStatementBuilder> incompleteStatements = new LinkedList<>();
  protected final Collection<CacheRefResolver> incompleteCacheRefs = new LinkedList<>();
  protected final Collection<ResultMapResolver> incompleteResultMaps = new LinkedList<>();
  protected final Collection<MethodResolver> incompleteMethods = new LinkedList<>();

这么多成员变量,先不一一解释作用,但是其中的几个参数我们应该是已经知道了的,第一个就是 mappedStatements ,上一篇我们知道被解析的mapper就是放在这里,后面的 resultMapsparameterMaps 也比较常用的就是我们参数和结果的映射map,这里跟我之前有一篇解释为啥我们一些变量的使用会比较特殊,比如list,可以参考这篇keyGenerators是在我们需要定义主键生成器的时候使用。
然后第二点是我们创建的 org.apache.ibatis.session.SqlSessionFactory 是哪个,

public SqlSessionFactory build(Configuration config) {
  return new DefaultSqlSessionFactory(config);
}

是这个 DefaultSqlSessionFactory ,这是其中一个 SqlSessionFactory 的实现
接下来我们看看 openSession 里干了啥

public SqlSession openSession() {
  return openSessionFromDataSource(configuration.getDefaultExecutorType(), null, false);
}

这边有几个参数,第一个是默认的执行器类型,往上找找上面贴着的 Configuration 的成员变量里可以看到默认是
protected ExecutorType defaultExecutorType = ExecutorType.SIMPLE;

因为没有指明特殊的执行逻辑,所以默认我们也就用简单类型的,第二个参数是是事务级别,第三个是是否自动提交

private SqlSession openSessionFromDataSource(ExecutorType execType, TransactionIsolationLevel level, boolean autoCommit) {
  Transaction tx = null;
  try {
    final Environment environment = configuration.getEnvironment();
    final TransactionFactory transactionFactory = getTransactionFactoryFromEnvironment(environment);
    tx = transactionFactory.newTransaction(environment.getDataSource(), level, autoCommit);
    // --------> 先关注这里
    final Executor executor = configuration.newExecutor(tx, execType);
    return new DefaultSqlSession(configuration, executor, autoCommit);
  } catch (Exception e) {
    closeTransaction(tx); // may have fetched a connection so lets call close()
    throw ExceptionFactory.wrapException("Error opening session.  Cause: " + e, e);
  } finally {
    ErrorContext.instance().reset();
  }
}

具体是调用了 Configuration 的这个方法

public Executor newExecutor(Transaction transaction, ExecutorType executorType) {
  executorType = executorType == null ? defaultExecutorType : executorType;
  Executor executor;
  if (ExecutorType.BATCH == executorType) {
    executor = new BatchExecutor(this, transaction);
  } else if (ExecutorType.REUSE == executorType) {
    executor = new ReuseExecutor(this, transaction);
  } else {
    // ---------> 会走到这个分支
    executor = new SimpleExecutor(this, transaction);
  }
  if (cacheEnabled) {
    executor = new CachingExecutor(executor);
  }
  executor = (Executor) interceptorChain.pluginAll(executor);
  return executor;
}

上面传入的 executorTypeConfiguration 的默认类型,也就是 simple 类型,并且 cacheEnabledConfiguration 默认为 true,所以会包装成CachingExecutor ,然后后面就是插件了,这块我们先不展开
然后我们的openSession返回的就是创建了DefaultSqlSession

public DefaultSqlSession(Configuration configuration, Executor executor, boolean autoCommit) {
    this.configuration = configuration;
    this.executor = executor;
    this.dirty = false;
    this.autoCommit = autoCommit;
  }

然后就是调用 selectOne, 因为前面已经把这部分代码说过了,就直接跳转过来
org.apache.ibatis.session.defaults.DefaultSqlSession#selectList(java.lang.String, java.lang.Object, org.apache.ibatis.session.RowBounds, org.apache.ibatis.session.ResultHandler)

private <E> List<E> selectList(String statement, Object parameter, RowBounds rowBounds, ResultHandler handler) {
  try {
    MappedStatement ms = configuration.getMappedStatement(statement);
    return executor.query(ms, wrapCollection(parameter), rowBounds, handler);
  } catch (Exception e) {
    throw ExceptionFactory.wrapException("Error querying database.  Cause: " + e, e);
  } finally {
    ErrorContext.instance().reset();
  }
}

因为前面说了 executor 包装了 CachingExecutor ,所以会先调用

@Override
public <E> List<E> query(MappedStatement ms, Object parameterObject, RowBounds rowBounds, ResultHandler resultHandler) throws SQLException {
  BoundSql boundSql = ms.getBoundSql(parameterObject);
  CacheKey key = createCacheKey(ms, parameterObject, rowBounds, boundSql);
  return query(ms, parameterObject, rowBounds, resultHandler, key, boundSql);
}

然后是调用的真实的query方法

@Override
public <E> List<E> query(MappedStatement ms, Object parameterObject, RowBounds rowBounds, ResultHandler resultHandler, CacheKey key, BoundSql boundSql)
    throws SQLException {
  Cache cache = ms.getCache();
  if (cache != null) {
    flushCacheIfRequired(ms);
    if (ms.isUseCache() && resultHandler == null) {
      ensureNoOutParams(ms, boundSql);
      @SuppressWarnings("unchecked")
      List<E> list = (List<E>) tcm.getObject(cache, key);
      if (list == null) {
        list = delegate.query(ms, parameterObject, rowBounds, resultHandler, key, boundSql);
        tcm.putObject(cache, key, list); // issue #578 and #116
      }
      return list;
    }
  }
  return delegate.query(ms, parameterObject, rowBounds, resultHandler, key, boundSql);
}

这里是第一次查询,没有缓存就先到最后一行,继续是调用到 org.apache.ibatis.executor.BaseExecutor#queryFromDatabase

@Override
  public <E> List<E> query(MappedStatement ms, Object parameter, RowBounds rowBounds, ResultHandler resultHandler, CacheKey key, BoundSql boundSql) throws SQLException {
    ErrorContext.instance().resource(ms.getResource()).activity("executing a query").object(ms.getId());
    if (closed) {
      throw new ExecutorException("Executor was closed.");
    }
    if (queryStack == 0 && ms.isFlushCacheRequired()) {
      clearLocalCache();
    }
    List<E> list;
    try {
      queryStack++;
      list = resultHandler == null ? (List<E>) localCache.getObject(key) : null;
      if (list != null) {
        handleLocallyCachedOutputParameters(ms, key, parameter, boundSql);
      } else {
        // ----------->会走到这里
        list = queryFromDatabase(ms, parameter, rowBounds, resultHandler, key, boundSql);
      }
    } finally {
      queryStack--;
    }
    if (queryStack == 0) {
      for (DeferredLoad deferredLoad : deferredLoads) {
        deferredLoad.load();
      }
      // issue #601
      deferredLoads.clear();
      if (configuration.getLocalCacheScope() == LocalCacheScope.STATEMENT) {
        // issue #482
        clearLocalCache();
      }
    }
    return list;
  }

然后是

private <E> List<E> queryFromDatabase(MappedStatement ms, Object parameter, RowBounds rowBounds, ResultHandler resultHandler, CacheKey key, BoundSql boundSql) throws SQLException {
  List<E> list;
  localCache.putObject(key, EXECUTION_PLACEHOLDER);
  try {
    list = doQuery(ms, parameter, rowBounds, resultHandler, boundSql);
  } finally {
    localCache.removeObject(key);
  }
  localCache.putObject(key, list);
  if (ms.getStatementType() == StatementType.CALLABLE) {
    localOutputParameterCache.putObject(key, parameter);
  }
  return list;
}

然后就是 simpleExecutor 的执行过程

@Override
public <E> List<E> doQuery(MappedStatement ms, Object parameter, RowBounds rowBounds, ResultHandler resultHandler, BoundSql boundSql) throws SQLException {
  Statement stmt = null;
  try {
    Configuration configuration = ms.getConfiguration();
    StatementHandler handler = configuration.newStatementHandler(wrapper, ms, parameter, rowBounds, resultHandler, boundSql);
    stmt = prepareStatement(handler, ms.getStatementLog());
    return handler.query(stmt, resultHandler);
  } finally {
    closeStatement(stmt);
  }
}

接下去其实就是跟jdbc交互了

@Override
public <E> List<E> query(Statement statement, ResultHandler resultHandler) throws SQLException {
  PreparedStatement ps = (PreparedStatement) statement;
  ps.execute();
  return resultSetHandler.handleResultSets(ps);
}

com.mysql.cj.jdbc.ClientPreparedStatement#execute

public boolean execute() throws SQLException {
        try {
            synchronized(this.checkClosed().getConnectionMutex()) {
                JdbcConnection locallyScopedConn = this.connection;
                if (!this.doPingInstead && !this.checkReadOnlySafeStatement()) {
                    throw SQLError.createSQLException(Messages.getString("PreparedStatement.20") + Messages.getString("PreparedStatement.21"), "S1009", this.exceptionInterceptor);
                } else {
                    ResultSetInternalMethods rs = null;
                    this.lastQueryIsOnDupKeyUpdate = false;
                    if (this.retrieveGeneratedKeys) {
                        this.lastQueryIsOnDupKeyUpdate = this.containsOnDuplicateKeyUpdate();
                    }

                    this.batchedGeneratedKeys = null;
                    this.resetCancelledState();
                    this.implicitlyCloseAllOpenResults();
                    this.clearWarnings();
                    if (this.doPingInstead) {
                        this.doPingInstead();
                        return true;
                    } else {
                        this.setupStreamingTimeout(locallyScopedConn);
                        Message sendPacket = ((PreparedQuery)this.query).fillSendPacket(((PreparedQuery)this.query).getQueryBindings());
                        String oldDb = null;
                        if (!locallyScopedConn.getDatabase().equals(this.getCurrentDatabase())) {
                            oldDb = locallyScopedConn.getDatabase();
                            locallyScopedConn.setDatabase(this.getCurrentDatabase());
                        }

                        CachedResultSetMetaData cachedMetadata = null;
                        boolean cacheResultSetMetadata = (Boolean)locallyScopedConn.getPropertySet().getBooleanProperty(PropertyKey.cacheResultSetMetadata).getValue();
                        if (cacheResultSetMetadata) {
                            cachedMetadata = locallyScopedConn.getCachedMetaData(((PreparedQuery)this.query).getOriginalSql());
                        }

                        locallyScopedConn.setSessionMaxRows(this.getQueryInfo().getFirstStmtChar() == 'S' ? this.maxRows : -1);
                        rs = this.executeInternal(this.maxRows, sendPacket, this.createStreamingResultSet(), this.getQueryInfo().getFirstStmtChar() == 'S', cachedMetadata, false);
                        if (cachedMetadata != null) {
                            locallyScopedConn.initializeResultsMetadataFromCache(((PreparedQuery)this.query).getOriginalSql(), cachedMetadata, rs);
                        } else if (rs.hasRows() && cacheResultSetMetadata) {
                            locallyScopedConn.initializeResultsMetadataFromCache(((PreparedQuery)this.query).getOriginalSql(), (CachedResultSetMetaData)null, rs);
                        }

                        if (this.retrieveGeneratedKeys) {
                            rs.setFirstCharOfQuery(this.getQueryInfo().getFirstStmtChar());
                        }

                        if (oldDb != null) {
                            locallyScopedConn.setDatabase(oldDb);
                        }

                        if (rs != null) {
                            this.lastInsertId = rs.getUpdateID();
                            this.results = rs;
                        }

                        return rs != null && rs.hasRows();
                    }
                }
            }
        } catch (CJException var11) {
            throw SQLExceptionsMapping.translateException(var11, this.getExceptionInterceptor());
        }
    }

前一篇讲了mybatis的初始化使用,如果我第一次看到这个使用入门文档,比较会产生疑惑的是配置了mapper,怎么就能通过selectOne跟语句id就能执行sql了,那么第一个问题,就是mapper是怎么被解析的,存在哪里,怎么被拿出来的

添加解析mapper

org.apache.ibatis.session.SqlSessionFactoryBuilder#build(java.io.InputStream)
public SqlSessionFactory build(InputStream inputStream) {
  return build(inputStream, null, null);
}

通过读取mybatis-config.xml来构建SqlSessionFactory,

public SqlSessionFactory build(InputStream inputStream, String environment, Properties properties) {
  try {
    // 创建下xml的解析器
    XMLConfigBuilder parser = new XMLConfigBuilder(inputStream, environment, properties);
    // 进行解析,后再构建
    return build(parser.parse());
  } catch (Exception e) {
    throw ExceptionFactory.wrapException("Error building SqlSession.", e);
  } finally {
    ErrorContext.instance().reset();
    try {
       if (inputStream != null) {
         inputStream.close();
       }
    } catch (IOException e) {
      // Intentionally ignore. Prefer previous error.
    }
  }

创建XMLConfigBuilder

public XMLConfigBuilder(InputStream inputStream, String environment, Properties props) {
    // --------> 创建 XPathParser
  this(new XPathParser(inputStream, true, props, new XMLMapperEntityResolver()), environment, props);
}

public XPathParser(InputStream inputStream, boolean validation, Properties variables, EntityResolver entityResolver) {
    commonConstructor(validation, variables, entityResolver);
    this.document = createDocument(new InputSource(inputStream));
  }

private XMLConfigBuilder(XPathParser parser, String environment, Properties props) {
  super(new Configuration());
  ErrorContext.instance().resource("SQL Mapper Configuration");
  this.configuration.setVariables(props);
  this.parsed = false;
  this.environment = environment;
  this.parser = parser;
}

这里主要是创建了Builder包含了Parser
然后调用parse方法

public Configuration parse() {
  if (parsed) {
    throw new BuilderException("Each XMLConfigBuilder can only be used once.");
  }
  // 标记下是否已解析,但是这里是否有线程安全问题
  parsed = true;
  // --------> 解析配置
  parseConfiguration(parser.evalNode("/configuration"));
  return configuration;
}

实际的解析区分了各类标签

private void parseConfiguration(XNode root) {
  try {
    // issue #117 read properties first
    // 解析properties,这个不是spring自带的,需要额外配置,并且在config文件里应该放在最前
    propertiesElement(root.evalNode("properties"));
    Properties settings = settingsAsProperties(root.evalNode("settings"));
    loadCustomVfs(settings);
    loadCustomLogImpl(settings);
    typeAliasesElement(root.evalNode("typeAliases"));
    pluginElement(root.evalNode("plugins"));
    objectFactoryElement(root.evalNode("objectFactory"));
    objectWrapperFactoryElement(root.evalNode("objectWrapperFactory"));
    reflectorFactoryElement(root.evalNode("reflectorFactory"));
    settingsElement(settings);
    // read it after objectFactory and objectWrapperFactory issue #631
    environmentsElement(root.evalNode("environments"));
    databaseIdProviderElement(root.evalNode("databaseIdProvider"));
    typeHandlerElement(root.evalNode("typeHandlers"));
    // ----------> 我们需要关注的是mapper的处理
    mapperElement(root.evalNode("mappers"));
  } catch (Exception e) {
    throw new BuilderException("Error parsing SQL Mapper Configuration. Cause: " + e, e);
  }
}

然后就是调用到mapperElement方法了

private void mapperElement(XNode parent) throws Exception {
  if (parent != null) {
    for (XNode child : parent.getChildren()) {
      if ("package".equals(child.getName())) {
        String mapperPackage = child.getStringAttribute("name");
        configuration.addMappers(mapperPackage);
      } else {
        String resource = child.getStringAttribute("resource");
        String url = child.getStringAttribute("url");
        String mapperClass = child.getStringAttribute("class");
        if (resource != null && url == null && mapperClass == null) {
          ErrorContext.instance().resource(resource);
          try(InputStream inputStream = Resources.getResourceAsStream(resource)) {
            XMLMapperBuilder mapperParser = new XMLMapperBuilder(inputStream, configuration, resource, configuration.getSqlFragments());
            // --------> 我们这没有指定package,所以是走到这
            mapperParser.parse();
          }
        } else if (resource == null && url != null && mapperClass == null) {
          ErrorContext.instance().resource(url);
          try(InputStream inputStream = Resources.getUrlAsStream(url)){
            XMLMapperBuilder mapperParser = new XMLMapperBuilder(inputStream, configuration, url, configuration.getSqlFragments());
            mapperParser.parse();
          }
        } else if (resource == null && url == null && mapperClass != null) {
          Class<?> mapperInterface = Resources.classForName(mapperClass);
          configuration.addMapper(mapperInterface);
        } else {
          throw new BuilderException("A mapper element may only specify a url, resource or class, but not more than one.");
        }
      }
    }
  }
}

核心就在这个parse()方法

public void parse() {
  if (!configuration.isResourceLoaded(resource)) {
    // -------> 然后就是走到这里,配置xml的mapper节点的内容
    configurationElement(parser.evalNode("/mapper"));
    configuration.addLoadedResource(resource);
    bindMapperForNamespace();
  }

  parsePendingResultMaps();
  parsePendingCacheRefs();
  parsePendingStatements();
}

具体的处理逻辑

private void configurationElement(XNode context) {
  try {
    String namespace = context.getStringAttribute("namespace");
    if (namespace == null || namespace.isEmpty()) {
      throw new BuilderException("Mapper's namespace cannot be empty");
    }
    builderAssistant.setCurrentNamespace(namespace);
    cacheRefElement(context.evalNode("cache-ref"));
    cacheElement(context.evalNode("cache"));
    parameterMapElement(context.evalNodes("/mapper/parameterMap"));
    resultMapElements(context.evalNodes("/mapper/resultMap"));
    sqlElement(context.evalNodes("/mapper/sql"));
    // ------->  走到这,从上下文构建statement
    buildStatementFromContext(context.evalNodes("select|insert|update|delete"));
  } catch (Exception e) {
    throw new BuilderException("Error parsing Mapper XML. The XML location is '" + resource + "'. Cause: " + e, e);
  }
}

具体代码在这,从上下文构建statement,只不过区分了下databaseId

private void buildStatementFromContext(List<XNode> list) {
  if (configuration.getDatabaseId() != null) {
    buildStatementFromContext(list, configuration.getDatabaseId());
  }
  // -----> 判断databaseId
  buildStatementFromContext(list, null);
}

判断下databaseId

private void buildStatementFromContext(List<XNode> list, String requiredDatabaseId) {
  for (XNode context : list) {
    final XMLStatementBuilder statementParser = new XMLStatementBuilder(configuration, builderAssistant, context, requiredDatabaseId);
    try {
      // -------> 解析statement节点
      statementParser.parseStatementNode();
    } catch (IncompleteElementException e) {
      configuration.addIncompleteStatement(statementParser);
    }
  }
}

接下来就是真正处理的xml语句内容的,各个节点的信息内容

public void parseStatementNode() {
  String id = context.getStringAttribute("id");
  String databaseId = context.getStringAttribute("databaseId");

  if (!databaseIdMatchesCurrent(id, databaseId, this.requiredDatabaseId)) {
    return;
  }

  String nodeName = context.getNode().getNodeName();
  SqlCommandType sqlCommandType = SqlCommandType.valueOf(nodeName.toUpperCase(Locale.ENGLISH));
  boolean isSelect = sqlCommandType == SqlCommandType.SELECT;
  boolean flushCache = context.getBooleanAttribute("flushCache", !isSelect);
  boolean useCache = context.getBooleanAttribute("useCache", isSelect);
  boolean resultOrdered = context.getBooleanAttribute("resultOrdered", false);

  // Include Fragments before parsing
  XMLIncludeTransformer includeParser = new XMLIncludeTransformer(configuration, builderAssistant);
  includeParser.applyIncludes(context.getNode());

  String parameterType = context.getStringAttribute("parameterType");
  Class<?> parameterTypeClass = resolveClass(parameterType);

  String lang = context.getStringAttribute("lang");
  LanguageDriver langDriver = getLanguageDriver(lang);

  // Parse selectKey after includes and remove them.
  processSelectKeyNodes(id, parameterTypeClass, langDriver);

  // Parse the SQL (pre: <selectKey> and <include> were parsed and removed)
  KeyGenerator keyGenerator;
  String keyStatementId = id + SelectKeyGenerator.SELECT_KEY_SUFFIX;
  keyStatementId = builderAssistant.applyCurrentNamespace(keyStatementId, true);
  if (configuration.hasKeyGenerator(keyStatementId)) {
    keyGenerator = configuration.getKeyGenerator(keyStatementId);
  } else {
    keyGenerator = context.getBooleanAttribute("useGeneratedKeys",
        configuration.isUseGeneratedKeys() && SqlCommandType.INSERT.equals(sqlCommandType))
        ? Jdbc3KeyGenerator.INSTANCE : NoKeyGenerator.INSTANCE;
  }

  // 语句的主要参数解析
  SqlSource sqlSource = langDriver.createSqlSource(configuration, context, parameterTypeClass);
  StatementType statementType = StatementType.valueOf(context.getStringAttribute("statementType", StatementType.PREPARED.toString()));
  Integer fetchSize = context.getIntAttribute("fetchSize");
  Integer timeout = context.getIntAttribute("timeout");
  String parameterMap = context.getStringAttribute("parameterMap");
  String resultType = context.getStringAttribute("resultType");
  Class<?> resultTypeClass = resolveClass(resultType);
  String resultMap = context.getStringAttribute("resultMap");
  String resultSetType = context.getStringAttribute("resultSetType");
  ResultSetType resultSetTypeEnum = resolveResultSetType(resultSetType);
  if (resultSetTypeEnum == null) {
    resultSetTypeEnum = configuration.getDefaultResultSetType();
  }
  String keyProperty = context.getStringAttribute("keyProperty");
  String keyColumn = context.getStringAttribute("keyColumn");
  String resultSets = context.getStringAttribute("resultSets");

  // --------> 添加映射的statement
  builderAssistant.addMappedStatement(id, sqlSource, statementType, sqlCommandType,
      fetchSize, timeout, parameterMap, parameterTypeClass, resultMap, resultTypeClass,
      resultSetTypeEnum, flushCache, useCache, resultOrdered,
      keyGenerator, keyProperty, keyColumn, databaseId, langDriver, resultSets);
}

添加的逻辑具体可以看下

public MappedStatement addMappedStatement(
    String id,
    SqlSource sqlSource,
    StatementType statementType,
    SqlCommandType sqlCommandType,
    Integer fetchSize,
    Integer timeout,
    String parameterMap,
    Class<?> parameterType,
    String resultMap,
    Class<?> resultType,
    ResultSetType resultSetType,
    boolean flushCache,
    boolean useCache,
    boolean resultOrdered,
    KeyGenerator keyGenerator,
    String keyProperty,
    String keyColumn,
    String databaseId,
    LanguageDriver lang,
    String resultSets) {

  if (unresolvedCacheRef) {
    throw new IncompleteElementException("Cache-ref not yet resolved");
  }

  id = applyCurrentNamespace(id, false);
  boolean isSelect = sqlCommandType == SqlCommandType.SELECT;

  MappedStatement.Builder statementBuilder = new MappedStatement.Builder(configuration, id, sqlSource, sqlCommandType)
      .resource(resource)
      .fetchSize(fetchSize)
      .timeout(timeout)
      .statementType(statementType)
      .keyGenerator(keyGenerator)
      .keyProperty(keyProperty)
      .keyColumn(keyColumn)
      .databaseId(databaseId)
      .lang(lang)
      .resultOrdered(resultOrdered)
      .resultSets(resultSets)
      .resultMaps(getStatementResultMaps(resultMap, resultType, id))
      .resultSetType(resultSetType)
      .flushCacheRequired(valueOrDefault(flushCache, !isSelect))
      .useCache(valueOrDefault(useCache, isSelect))
      .cache(currentCache);

  ParameterMap statementParameterMap = getStatementParameterMap(parameterMap, parameterType, id);
  if (statementParameterMap != null) {
    statementBuilder.parameterMap(statementParameterMap);
  }

  MappedStatement statement = statementBuilder.build();
  // ------>  正好是这里在configuration中添加了映射好的statement
  configuration.addMappedStatement(statement);
  return statement;
}

而里面就是往map里添加

public void addMappedStatement(MappedStatement ms) {
  mappedStatements.put(ms.getId(), ms);
}

获取mapper

StudentDO studentDO = session.selectOne("com.nicksxs.mybatisdemo.StudentMapper.selectStudent", 1);

就是调用了 org.apache.ibatis.session.defaults.DefaultSqlSession#selectOne(java.lang.String, java.lang.Object)

public <T> T selectOne(String statement, Object parameter) {
  // Popular vote was to return null on 0 results and throw exception on too many.
  List<T> list = this.selectList(statement, parameter);
  if (list.size() == 1) {
    return list.get(0);
  } else if (list.size() > 1) {
    throw new TooManyResultsException("Expected one result (or null) to be returned by selectOne(), but found: " + list.size());
  } else {
    return null;
  }
}

调用实际的实现方法

public <E> List<E> selectList(String statement, Object parameter) {
  return this.selectList(statement, parameter, RowBounds.DEFAULT);
}

这里还有一层

public <E> List<E> selectList(String statement, Object parameter, RowBounds rowBounds) {
  return selectList(statement, parameter, rowBounds, Executor.NO_RESULT_HANDLER);
}

根本的就是从configuration里获取了mappedStatement

private <E> List<E> selectList(String statement, Object parameter, RowBounds rowBounds, ResultHandler handler) {
  try {
    // 这里进行了获取
    MappedStatement ms = configuration.getMappedStatement(statement);
    return executor.query(ms, wrapCollection(parameter), rowBounds, handler);
  } catch (Exception e) {
    throw ExceptionFactory.wrapException("Error querying database.  Cause: " + e, e);
  } finally {
    ErrorContext.instance().reset();
  }
}

mybatis是我们比较常用的orm框架,下面是官网的介绍

MyBatis 是一款优秀的持久层框架,它支持自定义 SQL、存储过程以及高级映射。MyBatis 免除了几乎所有的 JDBC 代码以及设置参数和获取结果集的工作。MyBatis 可以通过简单的 XML 或注解来配置和映射原始类型、接口和 Java POJO(Plain Old Java Objects,普通老式 Java 对象)为数据库中的记录。

mybatis一大特点,或者说比较为人熟知的应该就是比 hibernate 是更轻量化,为国人所爱好的orm框架,对于hibernate目前还没有深入的拆解过,后续可以也写一下,在使用体验上觉得是个比较精巧的框架,看代码也比较容易,所以就想写个系列,第一篇先是介绍下使用
根据官网的文档上我们先来尝试一下简单使用
首先我们有个简单的配置,这个文件是mybatis-config.xml

<?xml version="1.0" encoding="UTF-8" ?>
<!DOCTYPE configuration
        PUBLIC "-//mybatis.org//DTD Config 3.0//EN"
        "https://mybatis.org/dtd/mybatis-3-config.dtd">
<configuration>
    <!-- 需要加入的properties-->
    <properties resource="application-development.properties"/>
    <!-- 指出使用哪个环境,默认是development-->
    <environments default="development">
        <environment id="development">
        <!-- 指定事务管理器类型-->
            <transactionManager type="JDBC"/>
            <!-- 指定数据源类型-->
            <dataSource type="POOLED">
                <!-- 下面就是具体的参数占位了-->
                <property name="driver" value="${driver}"/>
                <property name="url" value="${url}"/>
                <property name="username" value="${username}"/>
                <property name="password" value="${password}"/>
            </dataSource>
        </environment>
    </environments>
    <mappers>
        <!-- 指定mapper xml的位置或文件-->
        <mapper resource="mapper/StudentMapper.xml"/>
    </mappers>
</configuration>

在代码里创建mybatis里重要入口

String resource = "mybatis-config.xml";
InputStream inputStream = Resources.getResourceAsStream(resource);
SqlSessionFactory sqlSessionFactory = new SqlSessionFactoryBuilder().build(inputStream);

然后我们上面的StudentMapper.xml

<?xml version="1.0" encoding="UTF-8" ?>
<!DOCTYPE mapper
        PUBLIC "-//mybatis.org//DTD Mapper 3.0//EN"
        "https://mybatis.org/dtd/mybatis-3-mapper.dtd">
<mapper namespace="com.nicksxs.mybatisdemo.StudentMapper">
    <select id="selectStudent" resultType="com.nicksxs.mybatisdemo.StudentDO">
        select * from student where id = #{id}
    </select>
</mapper>

那么我们就要使用这个mapper,

String resource = "mybatis-config.xml";
InputStream inputStream = Resources.getResourceAsStream(resource);
SqlSessionFactory sqlSessionFactory = new SqlSessionFactoryBuilder().build(inputStream);
try (SqlSession session = sqlSessionFactory.openSession()) {
    StudentDO studentDO = session.selectOne("com.nicksxs.mybatisdemo.StudentMapper.selectStudent", 1);
    System.out.println("id is " + studentDO.getId() + " name is " +studentDO.getName());
} catch (Exception e) {
    e.printStackTrace();
}

sqlSessionFactory是sqlSession的工厂,我们可以通过sqlSessionFactory来创建sqlSession,而SqlSession 提供了在数据库执行 SQL 命令所需的所有方法。你可以通过 SqlSession 实例来直接执行已映射的 SQL 语句。可以看到mapper.xml中有定义mapper的namespace,就可以通过session.selectOne()传入namespace+id来调用这个方法
但是这样调用比较不合理的点,或者说按后面mybatis优化之后我们可以指定mapper接口

public interface StudentMapper {

    public StudentDO selectStudent(Long id);
}

就可以可以通过mapper接口获取方法,这样就不用涉及到未知的变量转换等异常

try (SqlSession session = sqlSessionFactory.openSession()) {
    StudentMapper mapper = session.getMapper(StudentMapper.class);
    StudentDO studentDO = mapper.selectStudent(1L);
    System.out.println("id is " + studentDO.getId() + " name is " +studentDO.getName());
} catch (Exception e) {
    e.printStackTrace();
}

这一篇咱们先介绍下简单的使用,后面可以先介绍下这些的原理。

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