asm 4.0 版本

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

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asm是java的字節碼操作框架，可以動態查看類的信息，動態修改，刪除，增加類的方法。

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下面基于4.0版本的一個使用示例，演示了對類Foo進行修改方法名稱，增加方法，修改方法內容等?

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);//新增加一個方法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)) { //當方法名為execute時，修改方法名為execute1return cv.visitMethod(access, name + "1", desc, signature, exceptions);}if("changeMethodContent".equals(name)) //此處的changeMethodContent即為需要修改的方法 ，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); //訪問需要修改的方法 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() { //此方法在訪問方法的頭部時被訪問到，僅被訪問一次//此處可插入新的指令super.visitCode();}@Overridepublic void visitInsn(int opcode) { //此方法可以獲取方法中每一條指令的操作類型，被訪問多次//如應在方法結尾處添加新指令，則應判斷：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);}}}

輸出：

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add方法是新增的，execute方法名改為execute1，changeMethodContent方法修改后增加了輸出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

我們把最終的字節碼保存到文件中e:\\logs\\Example.class中，再用反編譯工具java decompiler 查看最終的生成的源碼：

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最終的類如下：

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!");} }
接下來再慢慢研究asm里面對字節碼的操作，還有其他框架是如果使用asm的。

轉載于:https://www.cnblogs.com/zhwj184/archive/2012/08/13/3027473.html

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