这个问题也是面试中常被问到的，就抽空来了解下这个，跳过一大段前面初始化的逻辑，select * from t1 where id = #{id}这样的 sql，在初始化扫描 mapper 的xml文件的时候会根据是否是 dynamic 来判断生成 DynamicSqlSource 还是 RawSqlSource，这里它是一条 RawSqlSource，#{}替换成了?1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 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; } 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); if (textSqlNode.isDynamic()) { contents.add(textSqlNode); isDynamic = true ; } else { contents.add(new StaticTextSqlNode (data)); } } else if (child.getNode().getNodeType() == Node.ELEMENT_NODE) { 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); } public boolean isDynamic () { DynamicCheckerTokenParser checker = new DynamicCheckerTokenParser (); GenericTokenParser parser = createParser(checker); parser.parse(text); return checker.isDynamic(); } private GenericTokenParser createParser (TokenHandler handler) { return new GenericTokenParser ("${" , "}" , handler); }
可以看到其中一个条件就是是否有${}这种占位符，假如说上面的 sql 换成 ${}，那么可以看到它会在这里创建一个 dynamicSqlSource,1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 public class DynamicSqlSource implements SqlSource { private final Configuration configuration; private final SqlNode rootSqlNode; public DynamicSqlSource (Configuration configuration, SqlNode rootSqlNode) { this .configuration = configuration; this .rootSqlNode = rootSqlNode; } @Override public BoundSql getBoundSql (Object parameterObject) { DynamicContext context = new DynamicContext (configuration, parameterObject); rootSqlNode.apply(context); SqlSourceBuilder sqlSourceParser = new SqlSourceBuilder (configuration); Class<?> parameterType = parameterObject == null ? Object.class : parameterObject.getClass(); SqlSource sqlSource = sqlSourceParser.parse(context.getSql(), parameterType, context.getBindings()); BoundSql boundSql = sqlSource.getBoundSql(parameterObject); context.getBindings().forEach(boundSql::setAdditionalParameter); return boundSql; } }
这里眼尖的同学可能就可以看出来了，RawSqlSource 在初始化的时候已经经过了 parse，把#{}替换成了?占位符，但是 DynamicSqlSource 并没有#{}是在启动扫描 mapper的 xml 文件就替换成了 ?,然后是在什么时候变成实际的值的呢
再看这个图，我们发现在这的时候还没有进行替换
好像是这里了
