动态代理的具体使用
- 接口：
public interface UserService {
public void save();
public void delete();
public void update();
}
接口实现类【被代理类】：
public class UserServiceImpl implements UserService {
@Override
public void save() {
System.out.println("open 事务");
System.out.println("保存用户");
System.out.println("提交事务");
}
@Override
public void delete() {
System.out.println("open 事务");
System.out.println("删除用户");
System.out.println("提交事务");
}
@Override
public void update() {
System.out.println("open 事务");
System.out.println("更新用户信息");
System.out.println("提交事务");
}
}
方法增强类：
public class MyInvocationHandler implements InvocationHandler {
private Object object;
public MyInvocationHandler(Object object) {
this.object = object;
}
@Override
public Object invoke(Object proxy, Method method, Object[] args) throws Throwable {
System.out.println("+++++++before " + method.getName() + " +++++++++");
Object result = method.invoke(object, args);
System.out.println("++++++++after ");
return result;
}
}
动态代理实现类：
public class TestMain {
public static void main(String[] args) {
UserService userService = new UserServiceImpl();
InvocationHandler handler = new MyInvocationHandler(userService);
System.getProperties().setProperty("sun.misc.ProxyGenerator.saveGeneratedFiles", "true");
UserService proxy = (UserService)Proxy.newProxyInstance(userService.getClass().getClassLoader()
, userService.getClass().getInterfaces(), handler);
proxy.save();
proxy.delete();
}
}
实现结果：
+++++++before save +++++++++
open 事务
保存用户
提交事务
++++++++after
+++++++before delete +++++++++
open 事务
删除用户
提交事务
++++++++after
动态生成的代理类结果：
package com.sun.proxy;
import java.lang.reflect.InvocationHandler;
import java.lang.reflect.Method;
import java.lang.reflect.Proxy;
import java.lang.reflect.UndeclaredThrowableException;
import spring.boot.learn.service.UserService;
public final class $Proxy0 extends Proxy implements UserService {
private static Method m1;
private static Method m5;
private static Method m2;
private static Method m4;
private static Method m0;
private static Method m3;
public $Proxy0(InvocationHandler var1) throws {
super(var1);
}
public final boolean equals(Object var1) throws {
try {
return (Boolean)super.h.invoke(this, m1, new Object[]{var1});
} catch (RuntimeException | Error var3) {
throw var3;
} catch (Throwable var4) {
throw new UndeclaredThrowableException(var4);
}
}
public final void save() throws {
try {
super.h.invoke(this, m5, (Object[])null);
} catch (RuntimeException | Error var2) {
throw var2;
} catch (Throwable var3) {
throw new UndeclaredThrowableException(var3);
}
}
public final String toString() throws {
try {
return (String)super.h.invoke(this, m2, (Object[])null);
} catch (RuntimeException | Error var2) {
throw var2;
} catch (Throwable var3) {
throw new UndeclaredThrowableException(var3);
}
}
public final void delete() throws {
try {
super.h.invoke(this, m4, (Object[])null);
} catch (RuntimeException | Error var2) {
throw var2;
} catch (Throwable var3) {
throw new UndeclaredThrowableException(var3);
}
}
public final int hashCode() throws {
try {
return (Integer)super.h.invoke(this, m0, (Object[])null);
} catch (RuntimeException | Error var2) {
throw var2;
} catch (Throwable var3) {
throw new UndeclaredThrowableException(var3);
}
}
public final void update() throws {
try {
super.h.invoke(this, m3, (Object[])null);
} catch (RuntimeException | Error var2) {
throw var2;
} catch (Throwable var3) {
throw new UndeclaredThrowableException(var3);
}
}
static {
try {
m1 = Class.forName("java.lang.Object").getMethod("equals", Class.forName("java.lang.Object"));
m5 = Class.forName("spring.boot.learn.service.UserService").getMethod("save");
m2 = Class.forName("java.lang.Object").getMethod("toString");
m4 = Class.forName("spring.boot.learn.service.UserService").getMethod("delete");
m0 = Class.forName("java.lang.Object").getMethod("hashCode");
m3 = Class.forName("spring.boot.learn.service.UserService").getMethod("update");
} catch (NoSuchMethodException var2) {
throw new NoSuchMethodError(var2.getMessage());
} catch (ClassNotFoundException var3) {
throw new NoClassDefFoundError(var3.getMessage());
}
}
}
原理分析：
main 方法中主要通过 newProxyInstance 生成的代理类，看下如何实现的
```markup
public static Object newProxyInstance(ClassLoader loader,
Class<?>[] interfaces,
InvocationHandler h)
throws IllegalArgumentException
{
Objects.requireNonNull(h);
final Class<?>[] intfs = interfaces.clone();
final SecurityManager sm = System.getSecurityManager();
if (sm != null) {
checkProxyAccess(Reflection.getCallerClass(), loader, intfs);
}
/*
* Look up or generate the designated proxy class.
*/
Class<?> cl = getProxyClass0(loader, intfs);
/*
* Invoke its constructor with the designated invocation handler.
*/
try {
if (sm != null) {
checkNewProxyPermission(Reflection.getCallerClass(), cl);
}
final Constructor<?> cons = cl.getConstructor(constructorParams);
final InvocationHandler ih = h;
if (!Modifier.isPublic(cl.getModifiers())) {
AccessController.doPrivileged(new PrivilegedAction<Void>() {
public Void run() {
cons.setAccessible(true);
return null;
}
});
}
return cons.newInstance(new Object[]{h});
} catch (IllegalAccessException|InstantiationException e) {
throw new InternalError(e.toString(), e);
} catch (InvocationTargetException e) {
Throwable t = e.getCause();
if (t instanceof RuntimeException) {
throw (RuntimeException) t;
} else {
throw new InternalError(t.toString(), t);
}
} catch (NoSuchMethodException e) {
throw new InternalError(e.toString(), e);
}
}
newProxyInstance方法三个参数：类加载器，代理类实现的接口集合，方法增强类；主要的三个方法功能：
- 生成代理类的class对象并加载到内存【通过getProxyClass0方法实现】
- 确定用于初始化的的构造方法【proxy 可能生成多个构造方法，这儿确定使用带有 invokerhandler 参数的构造方法】【通过cl.getConstructor(constructorParams)方法实现】
- 初始化代理类对象构造方法入参是我们实现增强方法的 通过myInvokerHandler【cons.newInstance(new Object[]{h})方法实现】
看一下如何生成代理类的 class 对象：
public WeakCache(BiFunction<K, P, ?> subKeyFactory,
BiFunction<K, P, V> valueFactory) {
this.subKeyFactory = Objects.requireNonNull(subKeyFactory);
this.valueFactory = Objects.requireNonNull(valueFactory);
}
private static Class<?> getProxyClass0(ClassLoader loader,
Class<?>... interfaces) {
if (interfaces.length > 65535) {
throw new IllegalArgumentException("interface limit exceeded");
}
// If the proxy class defined by the given loader implementing
// the given interfaces exists, this will simply return the cached copy;
// otherwise, it will create the proxy class via the ProxyClassFactory
return proxyClassCache.get(loader, interfaces);
}
private static final WeakCache<ClassLoader, Class<?>[], Class<?>> proxyClassCache = new WeakCache<>(new KeyFactory(), new ProxyClassFactory());
getproxyclass0的俩入参：代理对象的类加载器，代理对象实现的接口的集合
方法的第一步判断了代理对象实现的接口的个数限制不能超过65535
第二步是通过缓存获取 class 对象：可以看到 proxyClassCache的结构是 WeakCache，初始化是俩对象KeyFactory（初始化subKeyFactory，用来生成缓存的key）ProxyClassFactory（初始化valueFactory，用来生成缓存的 val[代理类的 calss 对象]）
下面进入到 proxyClassCache的 get 方法：
public V get(K key, P parameter) {
Objects.requireNonNull(parameter);
expungeStaleEntries();
Object cacheKey = CacheKey.valueOf(key, refQueue);
// lazily install the 2nd level valuesMap for the particular cacheKey
ConcurrentMap<Object, Supplier<V>> valuesMap = map.get(cacheKey);
if (valuesMap == null) {
ConcurrentMap<Object, Supplier<V>> oldValuesMap
= map.putIfAbsent(cacheKey,
valuesMap = new ConcurrentHashMap<>());
if (oldValuesMap != null) {
valuesMap = oldValuesMap;
}
}
// create subKey and retrieve the possible Supplier<V> stored by that
// subKey from valuesMap
Object subKey = Objects.requireNonNull(subKeyFactory.apply(key, parameter));
Supplier<V> supplier = valuesMap.get(subKey);
Factory factory = null;
while (true) {
if (supplier != null) {
// supplier might be a Factory or a CacheValue<V> instance
V value = supplier.get();
if (value != null) {
return value;
}
}
// else no supplier in cache
// or a supplier that returned null (could be a cleared CacheValue
// or a Factory that wasn't successful in installing the CacheValue)
// lazily construct a Factory
if (factory == null) {
factory = new Factory(key, parameter, subKey, valuesMap);
}
if (supplier == null) {
supplier = valuesMap.putIfAbsent(subKey, factory);
if (supplier == null) {
// successfully installed Factory
supplier = factory;
}
// else retry with winning supplier
} else {
if (valuesMap.replace(subKey, supplier, factory)) {
// successfully replaced
// cleared CacheEntry / unsuccessful Factory
// with our Factory
supplier = factory;
} else {
// retry with current supplier
supplier = valuesMap.get(subKey);
}
}
}
}
可以看到subKey是用 subKeyFactory 这个类的 apply 方法生成【key 不重要，不再追进去看】；核心是 val 的生成，看一下 while 循环
while（true）死循环，只有内部获取到代理类的 class 对象才会停止，分析一下如何获取 class 对象：
由于是第一次生成代理类，通过subKey 无法获取代理类class 对象（suppler 为 null），第一个判断进不去，会生成 Factory对象，并把 facotry 赋值给 spullier，此时 supplier 不再是 null，循环进入第一个判断，执行 supplier.get()方法
看一下 get 方法是怎么执行的【supplier 是 factory 赋值的，所以看 factory 的 get 方法】：
public synchronized V get() { // serialize access
// re-check
Supplier<V> supplier = valuesMap.get(subKey);
if (supplier != this) {
// something changed while we were waiting:
// might be that we were replaced by a CacheValue
// or were removed because of failure ->
// return null to signal WeakCache.get() to retry
// the loop
return null;
}
// else still us (supplier == this)
// create new value
V value = null;
try {
value = Objects.requireNonNull(valueFactory.apply(key, parameter));
} finally {
if (value == null) { // remove us on failure
valuesMap.remove(subKey, this);
}
}
// the only path to reach here is with non-null value
assert value != null;
// wrap value with CacheValue (WeakReference)
CacheValue<V> cacheValue = new CacheValue<>(value);
// put into reverseMap
reverseMap.put(cacheValue, Boolean.TRUE);
// try replacing us with CacheValue (this should always succeed)
if (!valuesMap.replace(subKey, this, cacheValue)) {
throw new AssertionError("Should not reach here");
}
// successfully replaced us with new CacheValue -> return the value
// wrapped by it
return value;
}
前边给 WeakCache 初始化的时候valueFactory 是通过 new ProxyClassFactory【属于 Proxy 的静态内部类】实现的，追进去看看ProxyClassFactory的 apply 方法做了哪些事情：
public Class<?> apply(ClassLoader loader, Class<?>[] interfaces) {
Map<Class<?>, Boolean> interfaceSet = new IdentityHashMap<>(interfaces.length);
for (Class<?> intf : interfaces) {
/*
* Verify that the class loader resolves the name of this
* interface to the same Class object.
*/
Class<?> interfaceClass = null;
try {
interfaceClass = Class.forName(intf.getName(), false, loader);
} catch (ClassNotFoundException e) {
}
if (interfaceClass != intf) {
throw new IllegalArgumentException(
intf + " is not visible from class loader");
}
/*
* Verify that the Class object actually represents an
* interface.
*/
if (!interfaceClass.isInterface()) {
throw new IllegalArgumentException(
interfaceClass.getName() + " is not an interface");
}
/*
* Verify that this interface is not a duplicate.
*/
if (interfaceSet.put(interfaceClass, Boolean.TRUE) != null) {
throw new IllegalArgumentException(
"repeated interface: " + interfaceClass.getName());
}
}
String proxyPkg = null; // package to define proxy class in
int accessFlags = Modifier.PUBLIC | Modifier.FINAL;
/*
* Record the package of a non-public proxy interface so that the
* proxy class will be defined in the same package. Verify that
* all non-public proxy interfaces are in the same package.
*/
for (Class<?> intf : interfaces) {
int flags = intf.getModifiers();
if (!Modifier.isPublic(flags)) {
accessFlags = Modifier.FINAL;
String name = intf.getName();
int n = name.lastIndexOf('.');
String pkg = ((n == -1) ? "" : name.substring(0, n + 1));
if (proxyPkg == null) {
proxyPkg = pkg;
} else if (!pkg.equals(proxyPkg)) {
throw new IllegalArgumentException(
"non-public interfaces from different packages");
}
}
}
if (proxyPkg == null) {
// if no non-public proxy interfaces, use com.sun.proxy package
proxyPkg = ReflectUtil.PROXY_PACKAGE + ".";
}
/*
* Choose a name for the proxy class to generate.
*/
long num = nextUniqueNumber.getAndIncrement();
String proxyName = proxyPkg + proxyClassNamePrefix + num;
/*
* Generate the specified proxy class.
*/
byte[] proxyClassFile = ProxyGenerator.generateProxyClass(
proxyName, interfaces, accessFlags);
try {
return defineClass0(loader, proxyName,
proxyClassFile, 0, proxyClassFile.length);
} catch (ClassFormatError e) {
/*
* A ClassFormatError here means that (barring bugs in the
* proxy class generation code) there was some other
* invalid aspect of the arguments supplied to the proxy
* class creation (such as virtual machine limitations
* exceeded).
*/
throw new IllegalArgumentException(e.toString());
}
}
}
最主要的两行代码：
byte[] proxyClassFile = ProxyGenerator.generateProxyClass( proxyName, interfaces, accessFlags);
defineClass0(loader, proxyName,proxyClassFile, 0, proxyClassFile.length);
generateProxyClass 生成了代理类的字节码文件；入参是：代理类路径，实现的接口类，权限
进入方法内部可能看到generateClassFile 方法，这个方法生成了 Object 的hashCode，equeals，toString 等方法，实现了 interfaces 的方法，生成构造方法：
this.addProxyMethod(hashCodeMethod, Object.class);
this.addProxyMethod(equalsMethod, Object.class);
this.addProxyMethod(toStringMethod, Object.class);
Class[] var1 = this.interfaces;
int var2 = var1.length;
int var3;
Class var4;
for(var3 = 0; var3 < var2; ++var3) {
var4 = var1[var3];
Method[] var5 = var4.getMethods();
int var6 = var5.length;
for(int var7 = 0; var7 < var6; ++var7) {
Method var8 = var5[var7];
this.addProxyMethod(var8, var4);
}
}
private ProxyGenerator.MethodInfo generateConstructor() throws IOException {
ProxyGenerator.MethodInfo var1 = new ProxyGenerator.MethodInfo("<init>", "(Ljava/lang/reflect/InvocationHandler;)V", 1);
DataOutputStream var2 = new DataOutputStream(var1.code);
this.code_aload(0, var2);
this.code_aload(1, var2);
var2.writeByte(183);
var2.writeShort(this.cp.getMethodRef("java/lang/reflect/Proxy", "<init>", "(Ljava/lang/reflect/InvocationHandler;)V"));
var2.writeByte(177);
var1.maxStack = 10;
var1.maxLocals = 2;
var1.declaredExceptions = new short[0];
return var1;
}
上图生成构造方法时会把生成一个带有 InvocationHandler参数的构造方法；
截止到这里代理类的字节码的文件生成完成了，ProxyClassFactory.apply() 方法根据字节码文件调用defineClass0方法生成代理类的 class 文件，作为返回值返回到 proxy 类的newProxyInstance方法中（回到 proxy 的 newProxyInstance方法了）；
现在有个问题：proxy 代理类是如何跟 myInvocationHandler 关联起来的，又是如何调用的代理类的方法的？
在回头看生成的代理类文件的构造方法
public $Proxy0(InvocationHandler var1) throws {
super(var1);
}
newProxyInstance方法中初始化代理类的时候调用的是含有 InvocationHandler 参数的构造方法，入参是 myInvocationHandler 对象，初始化完成后父类（proxy)中的 h 对象的值就是 myInvocationHandler
当执行 save 方法时：
public final void save() throws {
try {
super.h.invoke(this, m5, (Object[])null);
} catch (RuntimeException | Error var2) {
throw var2;
} catch (Throwable var3) {
throw new UndeclaredThrowableException(var3);
}
}
会调用到proxy 的 h 对象 （myInvocationHandler） 中的 invoker 方法，myInvocationHandler 中的方法又会找到 UserServiceImpl 中的 save方法【为什么是 UserServiceImpl，可以看一下main 方法，里边做了初始化赋值】
end！
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