前言
最近学反射的时候了解了一下其应用——动态代理，觉得挺有意思，在此记录一些对动态代理的理解，并对源码进行简单的梳理
反射
浅析动态代理之前先复习一下什么是反射？
- 简单理解反射就是以类对象作为入口，能够获得类内所有内容的一种方式
- 比如调用某个类中的方法时有两种方式
- 常见的就是创建一个对象实例，然后调用其公共方法
- 另一种就是通过反射，先获取类对象，通过类对象根据方法名获取对应的方法，用invoke(obj,arg)的方式调用
- 在此就不进一步复习了，之所以先复习反射调用方法是因为理解动态代理要用
静态代理
理解动态代理之前先理解静态代理
直接丢例子吧
public static void staticProxyDemo() {
SuperMan superMan = new SuperMan();
superMan.say();
superMan.showMyName();
ProxySuperMan proxySuperMan = new ProxySuperMan();
proxySuperMan.say();
proxySuperMan.showMyName();
}
interface Human{
void showMyName();
void say();
}
class SuperMan implements Human{
@Override
public void showMyName() {
System.out.println("My name is SuperMan!");
}
@Override
public void say() {
System.out.println("Hello! I am SuperMan!");
}
}
class ProxySuperMan implements Human{
private SuperMan superMan = new SuperMan();
@Override
public void showMyName() {
System.out.println("我是proxy show");
superMan.showMyName();
}
@Override
public void say() {
System.out.println("我是proxy say");
superMan.say();
}
}
Hello! I am SuperMan!
My name is SuperMan!
我是proxy say
Hello! I am SuperMan!
我是proxy show
My name is SuperMan!
上述方式就是静态代理，可以实现以下功能
- ProxySuperMan能够提供和SuperMan一样名字的方法调用
- 可以实现SuperMan的同名方法一样的功能，并且还能在实现同名方法功能之前自定义一些方法（这就是最大的好处）
- 当我们需要在SuperMan的方法调用前后加一些功能时（在web项目中就比如验证功能），就可以通过代理的方式添加，而不需去改原类SuperMan的源码
动态代理
那么为什么要动态代理？
- 因为静态代理方法名都是写死的
- 如果要对多个类进行自定义意味着要自定义多个代理对象
- 比较麻烦
那么我们希望动态代理能实现什么功能呢？
- 我只需要定义一个代理类
- 当我想代理某个类的某个方法时，我只需要传入该类名以及方法名即可
- 并且实现一个公共的代理逻辑，如在调用代理类方法前先执行某个方法
为此我自己写了个简单的动态代理方式去实现上述效果
public static void myProxyDemo() throws NoSuchMethodException, IllegalAccessException, InvocationTargetException {
MyProxy myProxyForSuperMan = new MyProxy(Human.class,new SuperMan());
myProxyForSuperMan.invoke("say",null);
}
class MyProxy{
private Class clazz;
private Object obj;
public MyProxy(Class clazz,Object obj) {
this.clazz=clazz;
this.obj=obj;
}
public void invoke(String methodName, Object[] args) throws NoSuchMethodException, InvocationTargetException, IllegalAccessException {
Method method = clazz.getMethod(methodName);
System.out.println("before invoke ......");
method.invoke(obj,args);
System.out.println("after invoke ......");
}
}
before invoke ......
Hello! I am SuperMan!
after invoke ......
- 只需要在新建MyProxy传入需要代理的接口以及一个实例对象
- 就能够通过invoke传入方法名和参数实现对应方法的代理
- 代理的逻辑只需要自定义invoke即可
可以看到我自定义的方式虽然能够完成动态代理的功能，但是看着很low，并且并没有真正意义地新建一个代理类
那么我们看看如果用官方的Proxy是怎么进行代理的
public static void proxyDemo(){
HumanHandler humanHandler = new HumanHandler(new SuperMan());
Human proxy = (Human) Proxy.newProxyInstance(SuperMan.class.getClassLoader(), new Class[]{Human.class}, humanHandler);
proxy.showMyName();
proxy.say();
}
class HumanHandler implements InvocationHandler{
Object obj;
public HumanHandler(Object obj) {
this.obj = obj;
}
@Override
public Object invoke(Object proxy, Method method, Object[] args) throws Throwable {
System.out.println("before method: "+method.getName());
method.invoke(obj,args);
System.out.println("after method: "+method.getName());
return null;
}
}
before method: showMyName
My name is SuperMan!
after method: showMyName
before method: say
Hello! I am SuperMan!
after method: say
- 该方法能够得到一个真正的代理类
- 并且该类能够调用被代理类的所有同名方法
- 调用同名方法会执行在Handler中自定义的invoke逻辑
本文总结重点并不是怎么用，而是搞清楚Proxy怎么做到上述功能的
- 对比Proxy和我的方法，其实都是利用了invoke的方式传入对象和方法进行调用
- 从本质上没区别，代理的逻辑很简单
我在看源码前好奇的是，Proxy是怎么做到能够生成一个代理类并且该代理类拥有被代理类所有同名方法的，而且还能实现invoke逻辑而不是像我那样只能通过传入名字来调，带着这个问题我就去阅读源码
先进入 Proxy.newProxyInstance()
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);
}
}
- 根据方法名很容易知道该方法做了什么事情
-
cl = getProxyClass0
-
cl.getConstructor()
-
cons.newInstance(new Object[]{h})
我们接着看看怎么获得代理类的
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);
}
-
proxyClassCache.get()
- 如果缓存中有代理类，则直接返回
- 没有则创建
- 也就意味着Proxy对应同一组参数只会生成的唯一一个代理类对象
那么我们接着关注缓存中未命中怎么创建代理类对象的，进入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);
}
}
}
}
- 只需要关注未命中的情况，即
factory==null
- 此时会
factory = new Factory()
- 接着到
V value = supplier.get();
- 这里的
supplier
factory
- 因此我们进入
Factory
get
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;
}
}
- 我们可以看到该方法还是被
synchronized
- 说明考虑到了线程安全，怎么考虑的？现在先不考虑
- 获取value值主要通过
value = Objects.requireNonNull(valueFactory.apply(key, parameter));
- 所以我们关注
valueFactory.apply(key, parameter)
- 由于
valueFactory
BiFunction<K, P, V>
- 所以我们去看传了什么给
valueFactory
- 可以看到
Factory
valueFactory
proxyClassCache
-
proxyClassCache
getProxyClass0
- 我们去看
proxyClassCache
private static final WeakCache<ClassLoader, Class<?>[], Class<?>>
proxyClassCache = new WeakCache<>(new KeyFactory(), new ProxyClassFactory());
- 通过
WeakCache
valueFactory
new ProxyClassFactory()
- 所以
valueFactory.apply(key, parameter)
ProxyClassFactory().apply()
- 我们接着看apply方法
@Override
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());
}
}
}
- 代码太多了
- 我们关注最核心的那就是
看到这里其实还是没有解决我阅读源码前提出的问题，即Proxy是怎么生成一个能够具备接口同名方法的代理类，并且调用invoke?
- 我想，答案应该就在
ProxyGenerator.generateProxyClass（）
- 所以我们接着看
为了更好地理解，我们从结果入手，即我们先试图调用该方法看会得到什么结果
private static void createProxyClassFile(){
String name = "ProxyHuman";
byte[] data = ProxyGenerator.generateProxyClass(name,new Class[]{Human.class});
FileOutputStream out =null;
try {
out = new FileOutputStream(name+".class");
System.out.println((new File("")).getAbsolutePath());
out.write(data);
} catch (FileNotFoundException e) {
e.printStackTrace();
} catch (IOException e) {
e.printStackTrace();
} finally {
if(null!=out) try {
out.close();
} catch (IOException e) {
e.printStackTrace();
}
}
}
H:\case_java8
- 我们在上述路径中打开生成的class文件
ProxyHuman.class
public final class ProxyHuman extends Proxy implements Human {
private static Method m1;
private static Method m2;
private static Method m4;
private static Method m3;
private static Method m0;
public ProxyHuman(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 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 showMyName() throws {
try {
super.h.invoke(this, m4, (Object[])null);
} catch (RuntimeException | Error var2) {
throw var2;
} catch (Throwable var3) {
throw new UndeclaredThrowableException(var3);
}
}
public final void say() throws {
try {
super.h.invoke(this, m3, (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);
}
}
static {
try {
m1 = Class.forName("java.lang.Object").getMethod("equals", Class.forName("java.lang.Object"));
m2 = Class.forName("java.lang.Object").getMethod("toString");
m4 = Class.forName("hx.learn.Human").getMethod("showMyName");
m3 = Class.forName("hx.learn.Human").getMethod("say");
m0 = Class.forName("java.lang.Object").getMethod("hashCode");
} catch (NoSuchMethodException var2) {
throw new NoSuchMethodError(var2.getMessage());
} catch (ClassNotFoundException var3) {
throw new NoClassDefFoundError(var3.getMessage());
}
}
}
- 可以看到确实就是通过这个方法生成了一个代理类的字节码文件
- 并且该class中除了接口的同名方法还有Object的方法
- 我们还可以看到每个方法都会调用invoke方法
super.h.invoke(this, m3, (Object[])null);
- 这意味着我们调用toString等Object方法时也会调用invoke
我们可以试试
public static void proxyDemo(){
HumanHandler humanHandler = new HumanHandler(new SuperMan());
Human proxy = (Human) Proxy.newProxyInstance(SuperMan.class.getClassLoader(), new Class[]{Human.class}, humanHandler);
// proxy.showMyName();
// proxy.say();
proxy.toString();
}
before method: toString
after method: toString
果然如此
那么现在相当于只是知道了代理类的结构，还没能解决怎么做到自动生成同名方法这一问题，我们需要去看
generateProxyClass（）
的实现
public static byte[] generateProxyClass(final String var0, Class<?>[] var1, int var2) {
ProxyGenerator var3 = new ProxyGenerator(var0, var1, var2);
final byte[] var4 = var3.generateClassFile();
if (saveGeneratedFiles) {
AccessController.doPrivileged(new PrivilegedAction<Void>() {
public Void run() {
try {
int var1 = var0.lastIndexOf(46);
Path var2;
if (var1 > 0) {
Path var3 = Paths.get(var0.substring(0, var1).replace('.', File.separatorChar));
Files.createDirectories(var3);
var2 = var3.resolve(var0.substring(var1 + 1, var0.length()) + ".class");
} else {
var2 = Paths.get(var0 + ".class");
}
Files.write(var2, var4, new OpenOption[0]);
return null;
} catch (IOException var4x) {
throw new InternalError("I/O exception saving generated file: " + var4x);
}
}
});
}
return var4;
}
- 我们可以看到主要方法在
generateClassFile()
由于该方法变量名很多而且没有语义，阅读比较困难。我们只需要去找能够解答疑惑的代码段即可，下面我直接贴出我认为能够解答疑惑的代码段
我们要解决的两个问题是
- 怎么自动生成方法名
- 怎么自动生成return并且调用invoke
怎么自动生成方法名
- 先简单说下主要逻辑
- 使用一个Map存放所有需要生成的方法名
- 遍历Map，把所有方法名write进字节码中
==================================
- 一开始先把三个Object的方法存入Map
this.addProxyMethod(hashCodeMethod, Object.class);
this.addProxyMethod(equalsMethod, Object.class);
this.addProxyMethod(toStringMethod, Object.class);
- 接着通过反射的
getMethods（）
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);
}
}
- 最后遍历Map进行
write
-
write
var15 = this.methods.iterator();
while(var15.hasNext()) {
ProxyGenerator.MethodInfo var21 = (ProxyGenerator.MethodInfo)var15.next();
var21.write(var14);
}
怎么自动生成调用invoke的return代码？
- 阅读时我直接全文搜索invoke
- 发现在
generateMethod()
而在
generateClassFile()
中调用了
generateMethod()
，并且最终也是加入Map
Iterator var15;
try {
this.methods.add(this.generateConstructor());
var11 = this.proxyMethods.values().iterator();
while(var11.hasNext()) {
var12 = (List)var11.next();
var15 = var12.iterator();
while(var15.hasNext()) {
ProxyGenerator.ProxyMethod var16 = (ProxyGenerator.ProxyMethod)var15.next();
this.fields.add(new ProxyGenerator.FieldInfo(var16.methodFieldName, "Ljava/lang/reflect/Method;", 10));
this.methods.add(var16.generateMethod());
}
}
至此，问题全部解决了
探索问题答案的过程还是比较有意思的
最后对流程做个简单总结吧
- Proxy.newInstance会先查看缓存是否有对应的代理类
- 命中则直接返回，通过反射调用代理类构造方法新建实例对象返回
- 不命中则创建
- 创建过程简单的说就是用一个ProxyFactory模拟人写代码的过程
- 先查看传入的接口，看看被代理类有哪些方法
- 将这些方法名记录下来
- 同时默认要把Object三个方法也记录下来
- 并且还要记录return应该写什么
- 全都记录好以后调用write写一个class出来就行了
最后的最后补充为什么
Factory
的
get
要加锁
- 因为get要操作缓存，缓存必须保证线程安全
下一步我打算去看看Spring的AOP源码，再进行一次巩固学习
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