硬核 | 基于ASM实现Java类与接口的动态代理
2022-11-15 09:46:05来源：冰河技术
asm是一款编写字节码的框架，熟练使用可以加深对字节码指令的掌握。
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Java动态代理是基于接口代理的，所以首先我们得定义一个公共接口。
现在代理用户接口，实现登陆逻辑和来打印登录的花费时间
public interface UserService { boolean login(String username, String password);}再来看看Proxy的使用方法，newProxyInstance方法需要传三个参数，第一个类加载器，第二个需要代理的接口数组，第三个参数是调用方法处理器，也是我们写代理逻辑的需要实现的接口。
实现InvocationHandler，判断传入的username 和password是否等于admin，而且打印调用方法耗时。
public class UserServiceInvocationHandler implements InvocationHandler { @Override public Object invoke(Object proxy, Method method, Object[] args) throws InvocationTargetException, IllegalAccessException { long start = System.currentTimeMillis(); System.out.println("invoke:" + proxy.getClass().getSimpleName() + "." + method.getName() + ":" + (System.currentTimeMillis() - start) + "ms"); return "admin".equals(args[0]) && "admin".equals(args[1]); }}生成代理类
import java.lang.reflect.Proxy;public class App { public static void main(String[] args) { UserService userServiceProxy = (UserService) Proxy.newProxyInstance(App.class.getClassLoader(), new Class[]{UserService.class}, new UserServiceInvocationHandler()); System.out.println(userServiceProxy.getClass()); System.out.println(userServiceProxy.login("admin", "admin")); System.out.println(userServiceProxy.login("admin", "admin1")); }}调用main方法，打印结果
使用ASM实现首先我们先看一下生成的代理类最终的样子
import java.lang.reflect.InvocationHandler;import java.lang.reflect.Method;import proxy.ASMProxy;import proxy.UserService;public class $Proxy0 extends ASMProxy implements UserService { public static Method _UserService_0 = Class.forName("proxy.UserService").getMethod("login", Class.forName("java.lang.String"), Class.forName("java.lang.String")); public $Proxy0(InvocationHandler var1) { super(var1); } public boolean login(String var1, String var2) throws Exception { return (Boolean)super.h.invoke(this, _UserService_0, new Object[]{var1, var2}); }}三个要点：
InvocationHandler保存在ASMProxy中。要实现的接口方法Method使用静态字段保存。实现接口方法内实际调用父类的InvocationHandler的invoke方法。具体看看实现步骤
ASMProxypackage proxy;import java.lang.reflect.Constructor;import java.lang.reflect.InvocationHandler;import java.util.concurrent.atomic.AtomicInteger;public class ASMProxy { protected InvocationHandler h; //代理类名计数器 private static final AtomicInteger PROXY_CNT = new AtomicInteger(0); private static final String PROXY_CLASS_NAME_PRE = "$Proxy"; public ASMProxy(InvocationHandler var1) { h = var1; } public static Object newProxyInstance(ClassLoader loader, Class[] interfaces, InvocationHandler h)throws Exception { //生成代理类Class Class proxyClass = generate(interfaces); Constructor constructor = proxyClass.getConstructor(InvocationHandler.class); return constructor.newInstance(h); } /** * 生成代理类Class * * @param interfaces * @return */ private static Class generate(Class[] interfaces) throws ClassNotFoundException { String proxyClassName = PROXY_CLASS_NAME_PRE + PROXY_CNT.getAndIncrement(); byte[] codes = ASMProxyFactory.generateClass(interfaces, proxyClassName); //使用自定义类加载器加载字节码 ASMClassLoader asmClassLoader = new ASMClassLoader(); asmClassLoader.add(proxyClassName, codes); return asmClassLoader.loadClass(proxyClassName); }}ASMProxy的主要功能一个是作为代理类需要继承的父类，接着提供一个和Proxy同样的静态方法newProxyInstance。newProxyInstance里面调用ASMProxyFactory生成字节码二进制流，然后调用自定义的类加载器来生成Class。最后反射生成代理类的实例，返回对象。
ASMProxyFactory
接着看看最核心的部分，ASMProxyFactory是怎样生成字节码的，分几个步骤：
创建初始化方法声明静态字段创建静态方法实现接口方法package proxy;import org.objectweb.asm.ClassWriter;import org.objectweb.asm.MethodVisitor;import org.objectweb.asm.Opcodes;import org.objectweb.asm.Type;import java.lang.reflect.InvocationHandler;import java.lang.reflect.Method;import java.util.Arrays;import java.util.stream.Collectors;public class ASMProxyFactory { private static final Integer DEFAULT_NUM = 1; public static byte[] generateClass(Class[] interfaces, String proxyClassName) { //创建一个ClassWriter对象，自动计算栈帧和局部变量表大小 ClassWriter cw = new ClassWriter(ClassWriter.COMPUTE_FRAMES | ClassWriter.COMPUTE_MAXS); //创建的java版本、访问标志、类名、父类、接口 cw.visit(Opcodes.V1_8, Opcodes.ACC_PUBLIC, proxyClassName, null, Type.getInternalName(ASMProxy.class), getInterfacesName(interfaces)); //创建 createInit(cw); //创建static addStatic(cw, interfaces); //创建 addClinit(cw, interfaces, proxyClassName); //实现接口方法 addInterfacesImpl(cw, interfaces, proxyClassName); cw.visitEnd(); return cw.toByteArray(); } private static String[] getInterfacesName(Class[] interfaces) { String[] interfacesName = new String[interfaces.length]; return Arrays.stream(interfaces).map(Type::getInternalName).collect(Collectors.toList()).toArray(interfacesName); } /** * 创建init方法 * 调用父类的构造方法 * 0 aload_0 * 1 aload_1 * 2 invokespecial #1 : (Ljava/lang/reflect/InvocationHandler;)V> * 5 return * * @param cw */ private static void createInit(ClassWriter cw) { MethodVisitor mv = cw.visitMethod(Opcodes.ACC_PUBLIC, "", "(Ljava/lang/reflect/InvocationHandler;)V", null, null); mv.visitCode(); //将this入栈 mv.visitVarInsn(Opcodes.ALOAD, 0); //将参数入栈 mv.visitVarInsn(Opcodes.ALOAD, 1); //调用父类初始化方法 mv.visitMethodInsn(Opcodes.INVOKESPECIAL, Type.getInternalName(ASMProxy.class), "", "(Ljava/lang/reflect/InvocationHandler;)V", false); // 返回 mv.visitInsn(Opcodes.RETURN); mv.visitMaxs(2, 2); mv.visitEnd(); } /** * 创建static字段 * * @param cw * @param interfaces */ private static void addStatic(ClassWriter cw, Class[] interfaces) { for (Class anInterface : interfaces) { for (int i = 0; i < anInterface.getMethods().length; i++) { String methodName = "_" + anInterface.getSimpleName() + "_" + i; cw.visitField(Opcodes.ACC_PUBLIC | Opcodes.ACC_STATIC, methodName, Type.getDescriptor(Method.class), null, null); } } } private static void addClinit(ClassWriter cw, Class[] interfaces, String proxyClassName) { //_UserService_0 = Class.forName("proxy.UserService").getMethod("login", String.class, String.class); MethodVisitor mv = cw.visitMethod(Opcodes.ACC_STATIC, "", "()V", null, null); mv.visitCode(); for (Class anInterface : interfaces) { for (int i = 0; i < anInterface.getMethods().length; i++) { Method method = anInterface.getMethods()[i]; String methodName = "_" + anInterface.getSimpleName() + "_" + i; mv.visitLdcInsn(anInterface.getName()); mv.visitMethodInsn(Opcodes.INVOKESTATIC, Type.getInternalName(Class.class), "forName", "(Ljava/lang/String;)Ljava/lang/Class;", false); mv.visitLdcInsn(method.getName()); if (method.getParameterCount() == 0) { mv.visitInsn(Opcodes.ACONST_NULL); } else { switch (method.getParameterCount()) { case 1: mv.visitInsn(Opcodes.ICONST_1); break; case 2: mv.visitInsn(Opcodes.ICONST_2); break; case 3: mv.visitInsn(Opcodes.ICONST_3); break; default: mv.visitVarInsn(Opcodes.BIPUSH, method.getParameterCount()); break; } mv.visitTypeInsn(Opcodes.ANEWARRAY, Type.getInternalName(Class.class)); for (int paramIndex = 0; paramIndex < method.getParameterTypes().length; paramIndex++) { Class parameter = method.getParameterTypes()[paramIndex]; mv.visitInsn(Opcodes.DUP); switch (paramIndex) { case 0: mv.visitInsn(Opcodes.ICONST_0); break; case 1: mv.visitInsn(Opcodes.ICONST_1); break; case 2: mv.visitInsn(Opcodes.ICONST_2); break; case 3: mv.visitInsn(Opcodes.ICONST_3); break; default: mv.visitVarInsn(Opcodes.BIPUSH, paramIndex); break; } mv.visitLdcInsn(parameter.getName()); mv.visitMethodInsn( Opcodes.INVOKESTATIC, Type.getInternalName(Class.class), "forName", "(Ljava/lang/String;)Ljava/lang/Class;", false ); mv.visitInsn(Opcodes.AASTORE); } }// invokevirtual #13 mv.visitMethodInsn(Opcodes.INVOKEVIRTUAL, Type.getInternalName(Class.class), "getMethod", "(Ljava/lang/String;[Ljava/lang/Class;)Ljava/lang/reflect/Method;", false); //putstatic #3 mv.visitFieldInsn(Opcodes.PUTSTATIC, proxyClassName, methodName, Type.getDescriptor(Method.class)); } mv.visitInsn(Opcodes.RETURN); } mv.visitMaxs(DEFAULT_NUM, DEFAULT_NUM); mv.visitEnd(); } private static void addInterfacesImpl(ClassWriter cw, Class[] interfaces, String proxyClassName) { for (Class anInterface : interfaces) { for (int i = 0; i < anInterface.getMethods().length; i++) { Method method = anInterface.getMethods()[i]; String methodName = "_" + anInterface.getSimpleName() + "_" + i; MethodVisitor mv = cw.visitMethod(Opcodes.ACC_PUBLIC, method.getName(), Type.getMethodDescriptor(method), null, new String[]{Type.getInternalName(Exception.class)}); mv.visitCode(); mv.visitVarInsn(Opcodes.ALOAD, 0); mv.visitFieldInsn(Opcodes.GETFIELD, Type.getInternalName(ASMProxy.class), "h", "Ljava/lang/reflect/InvocationHandler;"); mv.visitVarInsn(Opcodes.ALOAD, 0); mv.visitFieldInsn(Opcodes.GETSTATIC, proxyClassName, methodName, Type.getDescriptor(Method.class)); // switch (method.getParameterCount()) { case 0: mv.visitInsn(Opcodes.ICONST_0); break; case 1: mv.visitInsn(Opcodes.ICONST_1); break; case 2: mv.visitInsn(Opcodes.ICONST_2); break; case 3: mv.visitInsn(Opcodes.ICONST_3); break; default: mv.visitVarInsn(Opcodes.BIPUSH, method.getParameterCount()); break; } mv.visitTypeInsn(Opcodes.ANEWARRAY, Type.getInternalName(Object.class)); // * 12 dup // * 13 iconst_0 // * 14 aload_1 // * 15 aastore for (int paramIndex = 0; paramIndex < method.getParameterCount(); paramIndex++) { mv.visitInsn(Opcodes.DUP); switch (paramIndex) { case 0: mv.visitInsn(Opcodes.ICONST_0); break; case 1: mv.visitInsn(Opcodes.ICONST_1); break; case 2: mv.visitInsn(Opcodes.ICONST_2); break; case 3: mv.visitInsn(Opcodes.ICONST_3); break; default: mv.visitVarInsn(Opcodes.BIPUSH, paramIndex); break; } mv.visitVarInsn(Opcodes.ALOAD, paramIndex + 1); mv.visitInsn(Opcodes.AASTORE); }// * 20 invokeinterface #5 count 4// * 25 checkcast #6 // * 28 invokevirtual #7 mv.visitMethodInsn(Opcodes.INVOKEINTERFACE, Type.getInternalName(InvocationHandler.class), "invoke", "(Ljava/lang/Object;Ljava/lang/reflect/Method;[Ljava/lang/Object;)Ljava/lang/Object;", true); addReturn(mv, method.getReturnType());// mv.visitFrame(Opcodes.F_FULL, 0, null, 0, null); mv.visitMaxs(DEFAULT_NUM, DEFAULT_NUM); mv.visitEnd(); } } } //添加方法返回 private static void addReturn(MethodVisitor mv, Class returnType) { if (returnType.isAssignableFrom(Void.class)) { mv.visitInsn(Opcodes.RETURN); return; } if (returnType.isAssignableFrom(boolean.class)) { //checkcast #6 // * 28 invokevirtual #7 mv.visitTypeInsn(Opcodes.CHECKCAST, Type.getInternalName(Boolean.class)); mv.visitMethodInsn(Opcodes.INVOKEVIRTUAL, Type.getInternalName(Boolean.class), "booleanValue", "()Z", false); mv.visitInsn(Opcodes.IRETURN); } else if (returnType.isAssignableFrom(int.class)) { mv.visitTypeInsn(Opcodes.CHECKCAST, Type.getInternalName(Integer.class)); mv.visitMethodInsn(Opcodes.INVOKEVIRTUAL, Type.getInternalName(Integer.class), "intValue", "()I", false); mv.visitInsn(Opcodes.IRETURN); } else if (returnType.isAssignableFrom(long.class)) { mv.visitTypeInsn(Opcodes.CHECKCAST, Type.getInternalName(Long.class)); mv.visitMethodInsn(Opcodes.INVOKEVIRTUAL, Type.getInternalName(Long.class), "longValue", "()J", false); mv.visitInsn(Opcodes.JRETURN); } else if (returnType.isAssignableFrom(short.class)) { mv.visitTypeInsn(Opcodes.CHECKCAST, Type.getInternalName(Short.class)); mv.visitMethodInsn(Opcodes.INVOKEVIRTUAL, Type.getInternalName(Short.class), "shortValue", "()S", false); mv.visitInsn(Opcodes.IRETURN); } else if (returnType.isAssignableFrom(byte.class)) { mv.visitTypeInsn(Opcodes.CHECKCAST, Type.getInternalName(Byte.class)); mv.visitMethodInsn(Opcodes.INVOKEVIRTUAL, Type.getInternalName(Byte.class), "byteValue", "()B", false); mv.visitInsn(Opcodes.IRETURN); } else if (returnType.isAssignableFrom(char.class)) { mv.visitTypeInsn(Opcodes.CHECKCAST, Type.getInternalName(Character.class)); mv.visitMethodInsn(Opcodes.INVOKEVIRTUAL, Type.getInternalName(Character.class), "charValue", "()C", false); mv.visitInsn(Opcodes.IRETURN); } else if (returnType.isAssignableFrom(float.class)) { mv.visitTypeInsn(Opcodes.CHECKCAST, Type.getInternalName(Float.class)); mv.visitMethodInsn(Opcodes.INVOKEVIRTUAL, Type.getInternalName(Float.class), "floatValue", "()F", false); mv.visitInsn(Opcodes.FRETURN); } else if (returnType.isAssignableFrom(double.class)) { mv.visitTypeInsn(Opcodes.CHECKCAST, Type.getInternalName(Double.class)); mv.visitMethodInsn(Opcodes.INVOKEVIRTUAL, Type.getInternalName(Double.class), "doubleValue", "()D", false); mv.visitInsn(Opcodes.DRETURN); } else { mv.visitTypeInsn(Opcodes.CHECKCAST, Type.getInternalName(returnType)); mv.visitInsn(Opcodes.ARETURN); } }} ASMClassLoader
自定义类加载器，提供add方法，添加<类名、字节码>映射关系。覆写findClass方法，当类名能找到对应字节码时，调用defineClass生成Class。
package proxy;import java.util.HashMap;import java.util.Map;public class ASMClassLoader extends ClassLoader { private final Map classMap = new HashMap<>(); @Override protected Class findClass(String name) throws ClassNotFoundException { if (classMap.containsKey(name)) { byte[] bytes = classMap.get(name); classMap.remove(name); return defineClass(name, bytes, 0, bytes.length); } return super.findClass(name); } public void add(String name, byte[] bytes) { classMap.put(name, bytes); }} App
package proxy;import java.lang.reflect.Proxy;public class App { public static void main(String[] args) throws Throwable { System.out.println("Java动态代理==========================="); UserService userServiceProxy = (UserService) Proxy.newProxyInstance(App.class.getClassLoader(), new Class[]{UserService.class}, new UserServiceInvocationHandler()); System.out.println(userServiceProxy.getClass()); System.out.println(userServiceProxy.login("admin", "admin")); System.out.println(userServiceProxy.login("admin", "admin1")); System.out.println("ASM动态代理==========================="); UserService userServiceAsm = (UserService) ASMProxy.newProxyInstance(App.class.getClassLoader(), new Class[]{UserService.class}, new UserServiceInvocationHandler()); System.out.println(userServiceAsm.getClass()); System.out.println(userServiceAsm.login("admin", "admin")); System.out.println(userServiceAsm.login("admin", "admin1")); }}运行App:打印两种代理方式的结果
