深入探索 Java 热部署

        ClassWriter cw = new ClassWriter(ClassWriter.<em>COMPUTE_MAXS</em>); 
        ClassReader cr = null;     
        String enhancedClassName = classSource.getEnhancedName(); 
        try { 
            cr = new ClassReader(new FileInputStream( 
                    classSource.getFile())); 
        } catch (IOException e) { 
            e.printStackTrace(); 
            return null; 
        } 
        ClassVisitor cv = new EnhancedModifier(cw, 
                className.replace(".", "/"), 
                enhancedClassName.replace(".", "/")); 
        cr.accept(cv, 0);

    public Class<?> redefineClass(String className){ 
        ClassWriter cw = new ClassWriter(ClassWriter.<em>COMPUTE_MAXS</em>); 
        ClassReader cr = null; 
        ClassSource cs = <em>classFiles</em>.get(className); 
        if(cs==null){ 
            return null; 
        } 
        try { 
            cr = new ClassReader(new FileInputStream(cs.getFile())); 
        } catch (IOException e) { 
            e.printStackTrace(); 
            return null; 
        } 
        ClassModifier cm = new ClassModifier(cw); 
        cr.accept(cm, 0); 
        byte[] code = cw.toByteArray(); 
        return defineClass(className, code, 0, code.length); 
 }

    // 在 class 文件发生改变时重新定义这个类
    private Class<?> redefineClass(String className, ClassSource classSource){ 
        ClassWriter cw = new ClassWriter(ClassWriter.<em>COMPUTE_MAXS</em>); 
        ClassReader cr = null; 
        classSource.update(); 
        String enhancedClassName = classSource.getEnhancedName();       
        try { 
            cr = new ClassReader( 
                    new FileInputStream(classSource.getFile())); 
        } catch (IOException e) { 
            e.printStackTrace(); 
            return null; 
        } 
        EnhancedModifier em = new EnhancedModifier(cw, className.replace(".", "/"), 
                enhancedClassName.replace(".", "/")); 
        ExtendModifier exm = new ExtendModifier(em, className.replace(".", "/"), 
                enhancedClassName.replace(".", "/")); 
        cr.accept(exm, 0); 
        byte[] code = cw.toByteArray(); 
        classSource.setByteCopy(code); 
        Class&lt;?&gt; clazz = defineClass(enhancedClassName, code, 0, code.length); 
        classSource.setClassCopy(clazz); 
        return clazz; 
 }

// 原始逻辑   
  Greeter p = new Greeter(); 
// 改变后的逻辑
  IGreeter p = (IGreeter)MyClassLoader.<em>getInstance</em>().
  findClass("com.example.Greeter").newInstance();

@Override 
public void visitTypeInsn(int opcode, String type) { 
	if(opcode==Opcodes.<em>NEW</em> &amp;&amp; type.equals(className)){ 
		List&lt;LocalVariableNode&gt; variables = node.localVariables; 
		String compileType = null; 
		for(int i=0;i&lt;variables.size();i++){ 
			LocalVariableNode localVariable = variables.get(i); 
			compileType = <em>formType</em>(localVariable.desc); 
			if(matchType(compileType)&amp;&amp;!valiableIndexUsed[i]){ 
				valiableIndexUsed[i] = true; 
				break; 
			} 
		} 
	mv.visitMethodInsn(Opcodes.<em>INVOKESTATIC</em>, <em>CLASSLOAD_TYPE</em>, 
	    "getInstance", "()L"+<em>CLASSLOAD_TYPE</em>+";"); 
	mv.visitLdcInsn(type.replace("/", ".")); 
	mv.visitMethodInsn(Opcodes.<em>INVOKEVIRTUAL</em>, <em>CLASSLOAD_TYPE</em>, 
	    "findClass", "(Ljava/lang/String;)Ljava/lang/Class;"); 
	mv.visitMethodInsn(Opcodes.<em>INVOKEVIRTUAL</em>, "java/lang/Class", 
	    "newInstance", "()Ljava/lang/Object;"); 
	mv.visitTypeInsn(Opcodes.<em>CHECKCAST</em>, compileType); 
	flag = true; 
	} else { 
		mv.visitTypeInsn(opcode, type); 
	} 
 }

 public class ReloadAgent { 
    public static void premain(String agentArgs, Instrumentation inst){ 
        GeneralTransformer trans = new GeneralTransformer(); 
        inst.addTransformer(trans); 
    } 
 }

 manifest-Version: 1.0 
 Premain-Class: com.example.ReloadAgent 
 Can-Redefine-Classes: true

@Override 
public byte [] transform(ClassLoader paramClassLoader, String paramString, 
     Class<?> paramClass, ProtectionDomain paramProtectionDomain, 
     byte [] paramArrayOfByte) throws IllegalClassFormatException { 
	String className = paramString.replace("/", "."); 
	if(className.equals("com.example.Test")){ 
		MyClassLoader cl = MyClassLoader.<em>getInstance</em>(); 
		cl.defineReference(className, "com.example.Greeter"); 
		return cl.getByteCode(className); 
	}else if(className.equals("com.example.Greeter")){ 
		MyClassLoader cl = MyClassLoader.<em>getInstance</em>(); 
		cl.redefineClass(className); 
		return cl.getByteCode(className); 
	} 
	return null; 
 }