文章目录
动态代理
用一个简单的例子来描述动态代理，你想租房子，一般的话，你需要四处找房子，很辛苦，你想在家躺着交了钱就行，所以你找了个代理（中介），代理去找好了房子，和房东商量，你过来看下房子签合同交钱就好了。这就是代理的作用。（找中介需谨慎，中介不同于代码，代码不会骗人）
实际代码中，当你有一个已有的方法，你在不希望修改它的前提下想要扩展它的功能，即可使用动态代理。典型案例就是Spring AOP。
例子代码
/**
* @author ddd
* @create 2021-06-25 19:21
* #desc 动态代理需要一个行为接口
**/
public interface action {
public void helloworld();
}
/**
* @author ddd
* @create 2021-06-25
* #desc 代理类
* **/
public class agent implements InvocationHandler {
private action action;
public agent(action action){
this.action=action;
}
@Override
public Object invoke(Object proxy, Method method, Object[] args) throws Throwable {
System.out.println(" 辛苦找房！ " );
Object getmethod=method.invoke(action,args);
return getmethod;
}
}
/**
* @author ddd
* @create 2021-06-25
* @desc 顾客
**/
public class Custom implements action{
@Override
public void helloworld() {
System.out.println(" 看房，签合同，交钱！ ");
}
}
/**
* @author ddd
* @create 2021-06-25
* @desc 运行类
**/
public class Main {
public static void main(String[] args) {
System.setProperty("sun.misc.ProxyGenerator.saveGeneratedFiles", "true");
action action=new Custom();
InvocationHandler handlerProxy=new agent(action);
/*
这一步会在内存中生成动态代理类
*/
action actionInstance = (My.DesignPattern.Agent.Jdk.action) Proxy.newProxyInstance(handlerProxy.getClass().getClassLoader(),
action.getClass().getInterfaces(),
handlerProxy);
actionInstance.helloworld();
}
}
Proxy.newProxyInstance()
大家都说动态代理很重要，用起来也很方便，被代理类实现一个行为接口，代理类实现InvocationHandler 接口，调用Proxy.newProxyInstance()即可生成一个代理类，那到底是怎么生成的代理类？我们进下源码Proxy.newProxyInstance()
public static Object newProxyInstance(ClassLoader loader,
Class<?>[] interfaces,
InvocationHandler h)
throws IllegalArgumentException
{
/**
* 传入的代理类非空
*/
Objects.requireNonNull(h);
// 使用克隆的方式拿到接口,新知识，什么是克隆？
final Class<?>[] intfs = interfaces.clone();
// 系统安全性检查
final SecurityManager sm = System.getSecurityManager();
if (sm != null) {
// check Proxy Access 看名字就可以了解，检查是否可以代理
checkProxyAccess(Reflection.getCallerClass(), loader, intfs);
}
/*
* Look up or generate the designated proxy class.
*
* 上面英文是源码注释
* 查找或者生成一个代理
* 哎！有意思，查找，或者，生成，说明有两种方式
*/
Class<?> cl = getProxyClass0(loader, intfs);
/*
* Invoke its constructor with the designated invocation handler.
*
* 源码注释：使用构造方法生成代理类
*/
try {
if (sm != null) {
checkNewProxyPermission(Reflection.getCallerClass(), cl);
}
/**
* 使用构造方法生成代理类
*/
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;
}
});
}
/**
* 返回new的对象
*/
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);
}
}
getProxyClass0(loader, intfs)
上面有一行代码的注释是查找或者生成一个代理类，我们来这个方法的源码
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
// 又封装一层，接着进入proxyClassCache.get
return proxyClassCache.get(loader, interfaces);
}
proxyClassCache.get(loader, interfaces)
public V get(K key, P parameter) {
Objects.requireNonNull(parameter);
expungeStaleEntries();
/**
* 从缓存中查找，咦，这就是查找一个代理类，但是我们还没有生成过
* 那在后续肯定有生成代理类放入缓存的操作，
* 利用缓存的话再次访问的速度就会很快
*
* key是什么，handlerProxy.getClass().getClassLoader()
*/
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生产者接口，函数型接口，有必要去了解一下
* 这里用于生产代理类
*/
Supplier<V> supplier = valuesMap.get(subKey);
Factory factory = null;
while (true) {
if (supplier != null) {
// supplier might be a Factory or a CacheValue<V> instance
/**
* 非空即可获得代理类
* 第一次进入valuesMap是空的，map.get()获取不到
* 所以循环条件是while（ture）
* 后面会将一个Factory赋值给supplier，然后通过factory.get()来获取
*/
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
/**
* 如果为空，就赋值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);
}
}
}
}
Factory:factory.get()
private final class Factory implements Supplier<V> {
private final K key;
private final P parameter;
private final Object subKey;
private final ConcurrentMap<Object, Supplier<V>> valuesMap;
Factory(K key, P parameter, Object subKey,
ConcurrentMap<Object, Supplier<V>> valuesMap) {
this.key = key;
this.parameter = parameter;
this.subKey = subKey;
this.valuesMap = valuesMap;
}
@Override
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 {
/**
* 关键代码只在这一句，valueFactory.apply(key, parameter)非空即可
* 再去看valueFactory.apply(key, parameter)
*/
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;
}
}
Proxy:ProxyClassFactory:apply()
private static final class ProxyClassFactory
implements BiFunction<ClassLoader, Class<?>[], Class<?>>
{
// prefix for all proxy class names
private static final String proxyClassNamePrefix = "$Proxy";
// next number to use for generation of unique proxy class names
private static final AtomicLong nextUniqueNumber = new AtomicLong();
@Override
public Class<?> apply(ClassLoader loader, Class<?>[] interfaces) {
/**
* 获取所有接口信息并便利
*/
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 {
/**
* Class.forName作用是要求JVM查找并加载指定的类,相当于将接口实例化
* ClassLoader loader是我们一开始传入的自己类的加载器
*/
interfaceClass = Class.forName(intf.getName(), false, loader);
} catch (ClassNotFoundException e) {
}
/**
* 判断当前类的实例化是否与我们创建的类是同一个加载器生成的
*/
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
// 代理类放在哪个路径下
// 接口的权限
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) {
/**
* 获取接口的权限
*/
int flags = intf.getModifiers();
/**
* 如果是public，则放在同一路径下
*/
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());
}
}
}
生成的代理类
以上过程就生成了代理类，那最终我们生成的代理类在哪呢？
在开头的例子中，我们在Main类中加了这么一行代码，它可以将我们生成的代理类保存到本地
System.setProperty("sun.misc.ProxyGenerator.saveGeneratedFiles", "true");
运行之后就可以看到
代理类的helloworld()
我们来看代理类的helloworld方法
public final class $Proxy0 extends Proxy implements action{
---
/*
而我们一开始在最初的main方法中，
InvocationHandler handlerProxy=new agent(action);
action actionInstance = (My.DesignPattern.Agent.Jdk.action)Proxy.newProxyInstance(handlerProxy.getClass().getClassLoader(),
action.getClass().getInterfaces(),
handlerProxy);
又通过$Proxy0构造方法传了new agent(action)，是InvocationHandler 的实现类
所以Proxy 类中的InvocationHandl就是我们写的代理类
*/
public $Proxy0(InvocationHandler var1) throws {
super(var1);
}
public final void helloworld() throws {
try {
/**
* 调用了父类的h变量的invoke，即agent类中的invoke
*/
super.h.invoke(this, m3, (Object[])null);
} catch (RuntimeException | Error var2) {
throw var2;
} catch (Throwable var3) {
throw new UndeclaredThrowableException(var3);
}
}
---
}
总结
1、动态代理基于反射，可以在程序运行时得到一个类的全部信息。
2、反射基于JVM的类加载机制和JVM内存模型中类，接口模板的存放。
3、动态代理通过以上技术在运行时动态生成一个代理对象。
4、为什么说是动态，因为agent传入的参数是action接口，可以在运行时传入任何实现了action接口的类来完成代理，使用更加灵活。
5、此文仅个人理解，并且因为反射涉及JVM知识，内容过多，未进一步深入。