HelloAsm（一）

Asm是一個可以被用來分析和修改java class文件的工具包。主要的類包括ClassReader, ClassVisitor, ClassAdapter等。

修改java class文件就涉及到字節碼的格式，類型定義等，下面是從網上找到的相關說明：

首先想到的，自然是嘗試訪問一個class文件的內容，使用下面的代碼可以實現打印一個類的信息：

ClassPrinter cp = new ClassPrinter();
		
		try {
			ClassReader cr = new ClassReader("java.lang.Runnable");
			cr.accept(cp, 0);
		} catch (IOException e) {
			e.printStackTrace();
		}

static class ClassPrinter implements ClassVisitor {

		@Override
		public void visit(int version, int access, String name, String signature, String superName, 
				String[] interfaces) {
			System.out.println("version " + version + "\n");
			System.out.println(name + " extends " + superName + " {");
		}

		@Override
		public AnnotationVisitor visitAnnotation(String arg0, boolean arg1) {
			return null;
		}

		@Override
		public void visitAttribute(Attribute arg0) {
		}

		@Override
		public void visitEnd() {
			System.out.println("}");
		}

		@Override
		public FieldVisitor visitField(int access, String name, String desc, String signature, 
				Object value) {
			System.out.println("" + desc + "" + name); 
			return null;
		}

		@Override
		public void visitInnerClass(String arg0, String arg1, String arg2, int arg3) {
		}

		@Override
		public MethodVisitor visitMethod(int access, String name, String desc, String signature,
				String[] exceptions) {
			System.out.println(name + desc);
			return null;
		}

		@Override
		public void visitOuterClass(String arg0, String arg1, String arg2) {
		}

		@Override
		public void visitSource(String arg0, String arg1) {
		}
		
	}

接下來，定義一個類：

ClassWriter cw = new ClassWriter(0);
		cw.visit(Opcodes.V1_7, Opcodes.ACC_PUBLIC + Opcodes.ACC_ABSTRACT + Opcodes.ACC_INTERFACE, 
				"Comparable", null, 
				"java/lang/Object", new String[] { "Mesurable"});
		
		cw.visitField(Opcodes.ACC_PUBLIC + Opcodes.ACC_STATIC + Opcodes.ACC_FINAL, 
				"LESS", "I", null, new Integer(-1)).visitEnd();
		cw.visitField(Opcodes.ACC_PUBLIC + Opcodes.ACC_STATIC + Opcodes.ACC_FINAL, "EQUAL", "I", 
				null, new Integer(0)).visitEnd();
		cw.visitField(Opcodes.ACC_PUBLIC + Opcodes.ACC_STATIC + Opcodes.ACC_FINAL, "GREATER", "I", 
				null, new Integer(1)).visitEnd();
		cw.visitMethod(Opcodes.ACC_PUBLIC + Opcodes.ACC_ABSTRACT, "compareTo", "(Ljava/lang/Object;)I",
				null, null).visitEnd();;// 權限，方法名，描述，參數對應泛型，異常（數組）
		
		cw.visitEnd();
		byte[] b = cw.toByteArray();
		
		MyclassLoader myClassLoader = new MyclassLoader();
		Class<?> c = myClassLoader.defineClass("Comparable", b);
		
		ClassReader cr1 = new ClassReader(b);
		cr1.accept(cp, 0);

最後，結合ClassAdapter，對一個已經存在的類進行修改（修改版本號爲V1_5）：

ClassWriter cw1 = new ClassWriter(0);
		ClassAdapter ca1 = new changeVersionAdapter(cw1);
		ClassReader cr2 = new ClassReader(b);
		cr2.accept(ca1, 0);
		
		ClassReader cr3 = new ClassReader(cw1.toByteArray());
		cr3.accept(cp, 0);

static class changeVersionAdapter extends ClassAdapter {
		

		public changeVersionAdapter(ClassVisitor cv) {
			super(cv);
		}

		@Override
		public void visit(int version, int access, String name, String signature, String superName, 
				String[] interfaces) {
			cv.visit(Opcodes.V1_5, access, name, signature, superName, interfaces);
		}
		
	}

時序圖：