Nicksxs's Blog

public @interface Author {
    String name();
    int year();
}

@Author(name = "nick", year = 2024)
public class Point {
    int x, y;
}

CtClass cc = ClassPool.getDefault().get("org.example.Point");

Object[] all = cc.getAnnotations();
Author a = (Author)all[0];

String name = a.name();
int year = a.year();
System.out.println("name: " + name + ", year: " + year);

name: nick, year: 2024

Process finished with exit code 0

public class DemoService {
    public List<String> queryList(String name, String title, String n3) {
        List<String> list = new ArrayList<>();
        list.add(name);
        list.add(title);
        return list;
    }
}

javassist.CtBehavior#insertBefore(java.lang.String)

javassist.CtBehavior#insertAfter(java.lang.String)

ClassPool pool = ClassPool.getDefault();
CtClass cc = pool.get(targetClass);
CtMethod method = cc.getDeclaredMethod(targetMethod);

method.insertBefore(
                    "{ java.lang.StringBuilder _paramValues = new StringBuilder();" +
                    "  Object[] params = $args;" +
                    "  if(params != null) {" +
                    "    for(int i=0; i<params.length; i++) {" +
                    "      if(i > 0) _paramValues.append(\", \");" +
                    "      _paramValues.append(String.valueOf(params[i]));" +
                    "    }" +
                    "  }" +
                    "  System.out.println(\"[Method Entry] " + targetMethod + " params: \" + _paramValues.toString());" +
                    "}"
            );

StringBuilder after = new StringBuilder();
    after.append("{")
            .append("    java.util.List _result = $_;\n")
            .append("    String _resultStr = (_result != null ? _result.toString() : \"null\");\n")
            .append("    int _size = (_result != null ? _result.size() : 0);\n")
            .append("    System.out.println(\"[Method Exit] " + targetMethod + " returns: \" + _resultStr + \" size: \" + _size);\n")
            .append("}");

public static void main(String[] args) throws Exception {
            // 假设要监控 org.example.DemoService 类中的 queryList 方法
            addMonitoringNew("org.example.DemoService", "queryList");
            // 2. 使用自定义类加载器加载增强后的类
            ClassLoader classLoader = new ClassLoader() {
                @Override
                public Class<?> loadClass(String name) throws ClassNotFoundException {
                    if (name.equals("org.example.DemoService")) {
                        try {
                            // 读取增强后的类文件
                            byte[] bytes = java.nio.file.Files.readAllBytes(
                                    java.nio.file.Paths.get("org/example/DemoService.class")
                            );
                            return defineClass(name, bytes, 0, bytes.length);
                        } catch (Exception e) {
                            throw new ClassNotFoundException("Failed to load enhanced class", e);
                        }
                    }
                    return super.loadClass(name);
                }
            };

            // 3. 加载增强后的类
            Class<?> enhancedClass = classLoader.loadClass("org.example.DemoService");

            // 4. 创建实例
            Object instance = enhancedClass.newInstance();

            // 5. 获取并调用方法
            Method processMethod = enhancedClass.getMethod("queryList", String.class, String.class, String.class);  // 假设方法接收一个String参数
            Object result = processMethod.invoke(instance, "name1", "title1", "n3");

            System.out.println("Method returned: " + result);
        }
}
可以看到我们的输出结果
```java
Successfully enhanced method: queryList
[Method Entry] queryList params: name1, title1, n3
[Method Exit] queryList returns: [name1, title1] size: 2
Method returned: [name1, title1]

$0, $1, $2, ...    	this and actual parameters
$args	An array of parameters. The type of $args is Object[].
$$	All actual parameters.
For example, m($$) is equivalent to m($1,$2,...)
 
$cflow(...)	cflow variable
$r	The result type. It is used in a cast expression.
$w	The wrapper type. It is used in a cast expression.
$_	The resulting value
$sig	An array of java.lang.Class objects representing the formal parameter types.
$type	A java.lang.Class object representing the formal result type.
$class	A java.lang.Class object representing the class currently edited.

public static void createEntity() throws Exception {
    ClassPool pool = ClassPool.getDefault();
    CtClass cl = pool.makeClass("Entity");
    CtConstructor cons = new CtConstructor(new CtClass[]{}, cl);
    cl.addConstructor(cons);
    cl.writeFile();
}

CtField param = new CtField(pool.get("java.lang.String"), "name", cl);
param.setModifiers(Modifier.PRIVATE);
cl.addField(param, CtField.Initializer.constant("ent1"));

cl.addMethod(CtNewMethod.setter("setName", param));
cl.addMethod(CtNewMethod.getter("getName", param));

CtConstructor con = new CtConstructor(new CtClass[]{}, cl);
// 需要转义下
con.setBody("{name = \"nick\";}");
cl.addConstructor(con);

con = new CtConstructor(new CtClass[]{pool.get("java.lang.String")}, cl);
// $0=this / $1,$2,$3...指的是参数，$1代表第一个参数
con.setBody("{$0.name = $1;}");
cl.addConstructor(con);

CtMethod ctMethod = new CtMethod(CtClass.voidType, "printName", new CtClass[]{}, cl);
ctMethod.setModifiers(Modifier.PUBLIC);
ctMethod.setBody("{System.out.println(name);}");
cl.addMethod(ctMethod);

public static void start(String input) {
    try {
        System.out.println("Demo starting ...");
        System.out.println(input);
        Thread.sleep(123);
    } catch (InterruptedException ignore) {
    }
}

MethodInfo methodInfo = declaredMethod.getMethodInfo();
CodeAttribute codeAttribute = methodInfo.getCodeAttribute();
LocalVariableAttribute attribute = (LocalVariableAttribute) codeAttribute.getAttribute(LocalVariableAttribute.tag);
System.out.println(attribute.variableName(0));

declaredMethod.insertAfter("System.out.println( " + attribute.variableName(0)+ " );");