asm 4.0 版本

http://forge.ow2.org/plugins/scmsvn/index.php?group_id=23

asm是java的字节码操作框架，可以动态查看类的信息，动态修改，删除，增加类的方法。

下面基于4.0版本的一个使用示例，演示了对类Foo进行修改方法名称，增加方法，修改方法内容等

import java.io.FileOutputStream;
import java.io.IOException;
import java.lang.reflect.InvocationTargetException;
import java.lang.reflect.Method;import org.objectweb.asm.ClassReader;
import org.objectweb.asm.ClassVisitor;
import org.objectweb.asm.ClassWriter;
import org.objectweb.asm.MethodVisitor;
import org.objectweb.asm.Opcodes;public class AsmExample extends ClassLoader implements Opcodes{public static  class Foo {public static void execute() {System.out.println("test changed method name");}public static void changeMethodContent() {System.out.println("test change method");}}public static void main(String[] args) throws IOException, IllegalArgumentException, SecurityException, IllegalAccessException, InvocationTargetException {ClassReader cr = new ClassReader(Foo.class.getName());ClassWriter cw = new ClassWriter(cr, ClassWriter.COMPUTE_MAXS);ClassVisitor cv = new MethodChangeClassAdapter(cw);cr.accept(cv, Opcodes.ASM4);//新增加一个方法MethodVisitor mw= cw.visitMethod(ACC_PUBLIC + ACC_STATIC,"add","([Ljava/lang/String;)V",null,null);// pushes the 'out' field (of type PrintStream) of the System classmw.visitFieldInsn(GETSTATIC,"java/lang/System","out","Ljava/io/PrintStream;");// pushes the "Hello World!" String constantmw.visitLdcInsn("this is add method print!");// invokes the 'println' method (defined in the PrintStream class)mw.visitMethodInsn(INVOKEVIRTUAL,"java/io/PrintStream","println","(Ljava/lang/String;)V");mw.visitInsn(RETURN);// this code uses a maximum of two stack elements and two local// variablesmw.visitMaxs(0, 0);mw.visitEnd();// gets the bytecode of the Example class, and loads it dynamicallybyte[] code = cw.toByteArray();AsmExample loader = new AsmExample();Class<?> exampleClass = loader.defineClass(Foo.class.getName(), code, 0, code.length);for(Method method:  exampleClass.getMethods()){System.out.println(method);}System.out.println("*************");// uses the dynamically generated class to print 'Helloworld'exampleClass.getMethods()[0].invoke(null, null);  //調用changeMethodContent，修改方法內容System.out.println("*************");exampleClass.getMethods()[1].invoke(null, null); //調用execute,修改方法名// gets the bytecode of the Example class, and loads it dynamicallyFileOutputStream fos = new FileOutputStream("e:\\logs\\Example.class");fos.write(code);fos.close();}static class MethodChangeClassAdapter extends ClassVisitor implements Opcodes {public MethodChangeClassAdapter(final ClassVisitor cv) {super(Opcodes.ASM4, cv);}@Overridepublic void visit(int version,int access,String name,String signature,String superName,String[] interfaces){if (cv != null) {cv.visit(version, access, name, signature, superName, interfaces);}}@Overridepublic MethodVisitor visitMethod(int access,String name,String desc,String signature,String[] exceptions){if (cv != null && "execute".equals(name)) { //当方法名为execute时，修改方法名为execute1return cv.visitMethod(access, name + "1", desc, signature, exceptions);}if("changeMethodContent".equals(name))  //此处的changeMethodContent即为需要修改的方法  ，修改方法內容{  MethodVisitor mv = cv.visitMethod(access, name, desc, signature, exceptions);//先得到原始的方法  MethodVisitor newMethod = null;  newMethod = new AsmMethodVisit(mv); //访问需要修改的方法  return newMethod;  }  if (cv != null) {return cv.visitMethod(access, name, desc, signature, exceptions);}return null;}}static  class AsmMethodVisit extends MethodVisitor {public AsmMethodVisit(MethodVisitor mv) {super(Opcodes.ASM4, mv);    }@Overridepublic void visitMethodInsn(int opcode, String owner, String name, String desc) {super.visitMethodInsn(opcode, owner, name, desc);}@Overridepublic void visitCode() {       //此方法在访问方法的头部时被访问到，仅被访问一次//此处可插入新的指令super.visitCode();}@Overridepublic void visitInsn(int opcode) {     //此方法可以获取方法中每一条指令的操作类型，被访问多次//如应在方法结尾处添加新指令，则应判断：if(opcode == Opcodes.RETURN){// pushes the 'out' field (of type PrintStream) of the System classmv.visitFieldInsn(GETSTATIC,"java/lang/System","out","Ljava/io/PrintStream;");// pushes the "Hello World!" String constantmv.visitLdcInsn("this is a modify method!");// invokes the 'println' method (defined in the PrintStream class)mv.visitMethodInsn(INVOKEVIRTUAL,"java/io/PrintStream","println","(Ljava/lang/String;)V");
//                mv.visitInsn(RETURN);}super.visitInsn(opcode);}}}

输出：

add方法是新增的，execute方法名改为execute1，changeMethodContent方法修改后增加了输出this is a modify method!

public static void AsmExample$Foo.changeMethodContent()
public static void AsmExample$Foo.execute1()
public static void AsmExample$Foo.add(java.lang.String[])
public native int java.lang.Object.hashCode()
public final native java.lang.Class java.lang.Object.getClass()
public final void java.lang.Object.wait(long,int) throws java.lang.InterruptedException
public final void java.lang.Object.wait() throws java.lang.InterruptedException
public final native void java.lang.Object.wait(long) throws java.lang.InterruptedException
public boolean java.lang.Object.equals(java.lang.Object)
public java.lang.String java.lang.Object.toString()
public final native void java.lang.Object.notify()
public final native void java.lang.Object.notifyAll()
*************
test change method
this is a modify method!
*************
test changed method name

我们把最终的字节码保存到文件中e:\\logs\\Example.class中，再用反编译工具java decompiler 查看最终的生成的源码：

最终的类如下：

import java.io.PrintStream;public class AsmExample$Foo
{public static void execute1(){System.out.println("test changed method name");}public static void changeMethodContent() {System.out.println("test change method");System.out.println("this is a modify method!");}public static void add(String[] paramArrayOfString){System.out.println("this is add method print!");}
}

接下来再慢慢研究asm里面对字节码的操作，还有其他框架是如果使用asm的。