设计模式之——代理模式
2019-03-06 本文已影响0人
Jerry_1116
1 代理模式的定义
代理模式：代理模式又叫委托模式，是为某个对象提供一个代理对象，并且由代理对象控制对原对象的访问。代理模式通俗来讲就是我们生活中常见的中介。
代理模式可以提供非常好的访问控制，应用比较广泛。
举个例子来说明：假如说我现在想买一辆二手车，虽然我可以自己去找车源，做质量检测等一系列的车辆过户流程，但是这确实太浪费我得时间和精力了。我只是想买一辆车而已为什么我还要额外做这么多事呢？于是我就通过中介公司来买车，他们来给我找车源，帮我办理车辆过户流程，我只是负责选择自己喜欢的车，然后付钱就可以了。
代理模式的通用类图：
代理模式的通用类图
- Subject
抽象主题角色：可以是抽象类，也可以是接口。抽象主题是一个普通的业务类型，无特殊要求。 - RealSubject
具体主题角色：也叫做被委托角色或被代理角色，是业务逻辑的具体执行者。 - Proxy
代理主题角色：也叫做委托类或代理类。它负责对真实角色的应用，把所有抽象主题类定义的方法限制委托给真实主题角色实现，并且在具体主题角色处理完毕前后做预处理和善后处理工作。
一个代理类可以代理多个被委托者或被代理者，因此一个代理类具体代理哪个具体主题角色，是由场景类决定的。最简单的情况是一个主题类和一个代理类。通常情况下，一个接口只需要一个代理类，具体代理哪个实现类有高层模块决定。
2 代理模式的优点
-
职责清晰
具体角色是实现具体的业务逻辑，不用关心其他非本职责的事务，通过后期的代理完成一件事务，代码清晰。在某些情况下，一个客户类不想或者不能直接引用一个委托对象，而代理类对象可以在客户类和委托对象之间起到中介的作用，其特征是代理类和委托类实现相同的接口。 -
高扩展性
具体主题角色随时会发生变化，但是只要实现了接口，接口不变，代理类就可以不做任何修改继续使用，符合“开闭原则”。
另外，代理类除了是客户类和委托类的中介之外，我们还可以通过给代理类增加额外的功能来扩展委托类的功能，这样做我们只需要修改代理类而不需要再修改委托类，同样符合开闭原则。 -
智能化待完善
动态代理的智能化
3 代理模式的使用场景
代理类主要负责为委托类预处理消息、过滤消息、把消息转发给委托类，以及事后对返回结果的处理等。代理类本身并不真正实现服务，而是通过调用委托类的相关方法，来提供特定的服务。真正的业务功能还是由委托类来实现，但是可以在业务功能执行的前后加入一些公共的服务。例如我们想给项目加入缓存、日志这些功能，我们就可以使用代理类来完成，而没必要打开已经封装好的委托类。
4 代理模式通用示例
为了保持行为的一致性，代理类和委托类通常会实现相同的接口，所以在访问者看来两者没有丝毫的区别。通过代理类这中间一层，能有效控制对委托类对象的直接访问，也可以很好地隐藏和保护委托类对象，同时也为实施不同控制策略预留了空间，从而在设计上获得了更大的灵活性。
更通俗的说，代理解决的问题是：当两个类需要通信时，引入第三方代理类，将两个类的关系解耦，让我们只了解代理类即可，而且代理的出现还可以让我们完成与另一个类之间的关系的统一管理。但是切记，代理类和委托类要实现相同的接口，因为代理真正调用的还是委托类的方法。
代理模式有多种不同的实现方式。如果按照代理创建的时期来进行分类：静态代理、动态代理。
- 静态代理
由程序员创建或特定工具自动生成源代码，再对其进行编译。在程序运行之前，代理类.class文件就已经被创建，代理类和委托类的关系在运行前就确定。 - 动态代理
动态代理类的源码是在程序运行期间由JVM根据反射等机制动态的生成，所以不存在代理类的字节码文件。代理类和委托类的关系是在程序运行时确定。
4.1 静态代理
- 抽象主题类
public interface Subject {
/**
* 接口方法
*/
public void request();
}
- 具体主题类
public class ConcreteSubject implements Subject {
/**
* 具体的业务逻辑实现
*/
@Override
public void request() {
//业务处理逻辑
}
}
- 代理类
public class Proxy implements Subject {
/**
* 要代理的实现类
*/
private Subject subject = null;
/**
* 默认代理自己
*/
public Proxy() {
this.subject = new Proxy();
}
public Proxy(Subject subject) {
this.subject = subject;
}
/**
* 构造函数，传递委托者
*
* @param objects 委托者
*/
public Proxy(Object... objects) {
}
/**
* 实现接口方法
*/
@Override
public void request() {
this.before();
this.subject.request();
this.after();
}
/**
* 预处理
*/
private void before() {
//do something
}
/**
* 后处理
*/
private void after() {
//do something
}
}
- 客户端类
public class Client {
public static void main(String[] args) {
Subject subject = new ConcreteSubject();
Proxy proxy = new Proxy(subject);
proxy.request();
}
}
- 静态代理优缺点
- 优点
业务类只需要关注业务逻辑本身，保证了业务类的重用性。这是代理的共有优点。代理使客户端不需要知道实现类是什么，怎么做的，而客户端只需知道代理即可（解耦合）。对于如上的客户端代码，Subject subject = new ConcreteSubject(); Proxy proxy = new Proxy(subject);可以应用工厂方法将它隐藏。 - 缺点
- 代理类和委托类实现了相同的接口，代理类通过委托类实现了相同的方法。这样就出现了大量的代码重复。如果接口增加一个方法，除了所有实现类需要实现这个方法外，所有代理类也需要实现此方法。增加了代码维护的复杂度。
- 代理对象只服务于一种类型的对象，如果要服务多类型的对象。势必要为每一种对象都进行代理，静态代理在程序规模稍大时就无法胜任了。如上的代码是只为Subject类的访问提供了代理，但是如果还要为其他类如AnotherSubject类提供代理的话，就需要我们再次添加代理AnotherSubject的代理类。
由于静态代理的这个缺点，就需要使用动态代理。
4.2 动态代理
从静态代理会发现——每个代理类只能为一个接口服务，这样程序开发中必然会产生许多的代理类。所以我们想办法通过一个代理类完成全部的代理功能，那么我们就需要用动态代理.
在上面4.1的示例中，一个代理只能代理一种类型，而且是在编译器就已经确定被代理的对象。而动态代理是在运行时，通过反射机制实现动态代理，并且能够代理各种类型的对象。
在Java中要想实现动态代理机制，需要java.lang.reflect.InvocationHandler接口和 java.lang.reflect.Proxy类的支持。
-
java.lang.reflect.InvocationHandler接口的定义
package java.lang.reflect;
public interface InvocationHandler {
/**
* Processes a method invocation on a proxy instance and returns
* the result. This method will be invoked on an invocation handler
* when a method is invoked on a proxy instance that it is
* associated with.
*
* @param proxy the proxy instance that the method was invoked on
*
* @param method the {@code Method} instance corresponding to
* the interface method invoked on the proxy instance. The declaring
* class of the {@code Method} object will be the interface that
* the method was declared in, which may be a superinterface of the
* proxy interface that the proxy class inherits the method through.
*
* @param args an array of objects containing the values of the
* arguments passed in the method invocation on the proxy instance,
* or {@code null} if interface method takes no arguments.
* Arguments of primitive types are wrapped in instances of the
* appropriate primitive wrapper class, such as
* {@code java.lang.Integer} or {@code java.lang.Boolean}.
*
* @return the value to return from the method invocation on the
* proxy instance. If the declared return type of the interface
* method is a primitive type, then the value returned by
* this method must be an instance of the corresponding primitive
* wrapper class; otherwise, it must be a type assignable to the
* declared return type. If the value returned by this method is
* {@code null} and the interface method's return type is
* primitive, then a {@code NullPointerException} will be
* thrown by the method invocation on the proxy instance. If the
* value returned by this method is otherwise not compatible with
* the interface method's declared return type as described above,
* a {@code ClassCastException} will be thrown by the method
* invocation on the proxy instance.
*
* @throws Throwable the exception to throw from the method
* invocation on the proxy instance. The exception's type must be
* assignable either to any of the exception types declared in the
* {@code throws} clause of the interface method or to the
* unchecked exception types {@code java.lang.RuntimeException}
* or {@code java.lang.Error}. If a checked exception is
* thrown by this method that is not assignable to any of the
* exception types declared in the {@code throws} clause of
* the interface method, then an
* {@link UndeclaredThrowableException} containing the
* exception that was thrown by this method will be thrown by the
* method invocation on the proxy instance.
*
* @see UndeclaredThrowableException
*/
public Object invoke(Object proxy, Method method, Object[] args)
throws Throwable;
}
参数解释：
- Object proxy
被代理对象 - Method method
要调用的方法 - Object[] args
方法调用时所需要的参数
-
java.lang.reflect.Proxy类的定义
public class Proxy implements java.io.Serializable {
private static final long serialVersionUID = -2222568056686623797L;
/** parameter types of a proxy class constructor */
private static final Class<?>[] constructorParams =
{ InvocationHandler.class };
/**
* a cache of proxy classes
*/
private static final WeakCache<ClassLoader, Class<?>[], Class<?>>
proxyClassCache = new WeakCache<>(new KeyFactory(), new ProxyClassFactory());
/**
* the invocation handler for this proxy instance.
* @serial
*/
protected InvocationHandler h;
/**
* Prohibits instantiation.
*/
private Proxy() {
}
/**
* Constructs a new {@code Proxy} instance from a subclass
* (typically, a dynamic proxy class) with the specified value
* for its invocation handler.
*
* @param h the invocation handler for this proxy instance
*
* @throws NullPointerException if the given invocation handler, {@code h},
* is {@code null}.
*/
protected Proxy(InvocationHandler h) {
Objects.requireNonNull(h);
this.h = h;
}
/**
* Returns the {@code java.lang.Class} object for a proxy class
* given a class loader and an array of interfaces. The proxy class
* will be defined by the specified class loader and will implement
* all of the supplied interfaces. If any of the given interfaces
* is non-public, the proxy class will be non-public. If a proxy class
* for the same permutation of interfaces has already been defined by the
* class loader, then the existing proxy class will be returned; otherwise,
* a proxy class for those interfaces will be generated dynamically
* and defined by the class loader.
*
* <p>There are several restrictions on the parameters that may be
* passed to {@code Proxy.getProxyClass}:
*
* <ul>
* <li>All of the {@code Class} objects in the
* {@code interfaces} array must represent interfaces, not
* classes or primitive types.
*
* <li>No two elements in the {@code interfaces} array may
* refer to identical {@code Class} objects.
*
* <li>All of the interface types must be visible by name through the
* specified class loader. In other words, for class loader
* {@code cl} and every interface {@code i}, the following
* expression must be true:
* <pre>
* Class.forName(i.getName(), false, cl) == i
* </pre>
*
* <li>All non-public interfaces must be in the same package;
* otherwise, it would not be possible for the proxy class to
* implement all of the interfaces, regardless of what package it is
* defined in.
*
* <li>For any set of member methods of the specified interfaces
* that have the same signature:
* <ul>
* <li>If the return type of any of the methods is a primitive
* type or void, then all of the methods must have that same
* return type.
* <li>Otherwise, one of the methods must have a return type that
* is assignable to all of the return types of the rest of the
* methods.
* </ul>
*
* <li>The resulting proxy class must not exceed any limits imposed
* on classes by the virtual machine. For example, the VM may limit
* the number of interfaces that a class may implement to 65535; in
* that case, the size of the {@code interfaces} array must not
* exceed 65535.
* </ul>
*
* <p>If any of these restrictions are violated,
* {@code Proxy.getProxyClass} will throw an
* {@code IllegalArgumentException}. If the {@code interfaces}
* array argument or any of its elements are {@code null}, a
* {@code NullPointerException} will be thrown.
*
* <p>Note that the order of the specified proxy interfaces is
* significant: two requests for a proxy class with the same combination
* of interfaces but in a different order will result in two distinct
* proxy classes.
*
* @param loader the class loader to define the proxy class
* @param interfaces the list of interfaces for the proxy class
* to implement
* @return a proxy class that is defined in the specified class loader
* and that implements the specified interfaces
* @throws IllegalArgumentException if any of the restrictions on the
* parameters that may be passed to {@code getProxyClass}
* are violated
* @throws SecurityException if a security manager, <em>s</em>, is present
* and any of the following conditions is met:
* <ul>
* <li> the given {@code loader} is {@code null} and
* the caller's class loader is not {@code null} and the
* invocation of {@link SecurityManager#checkPermission
* s.checkPermission} with
* {@code RuntimePermission("getClassLoader")} permission
* denies access.</li>
* <li> for each proxy interface, {@code intf},
* the caller's class loader is not the same as or an
* ancestor of the class loader for {@code intf} and
* invocation of {@link SecurityManager#checkPackageAccess
* s.checkPackageAccess()} denies access to {@code intf}.</li>
* </ul>
* @throws NullPointerException if the {@code interfaces} array
* argument or any of its elements are {@code null}
*/
@CallerSensitive
public static Class<?> getProxyClass(ClassLoader loader,
Class<?>... interfaces)
throws IllegalArgumentException
{
final Class<?>[] intfs = interfaces.clone();
final SecurityManager sm = System.getSecurityManager();
if (sm != null) {
checkProxyAccess(Reflection.getCallerClass(), loader, intfs);
}
return getProxyClass0(loader, intfs);
}
/*
* Check permissions required to create a Proxy class.
*
* To define a proxy class, it performs the access checks as in
* Class.forName (VM will invoke ClassLoader.checkPackageAccess):
* 1. "getClassLoader" permission check if loader == null
* 2. checkPackageAccess on the interfaces it implements
*
* To get a constructor and new instance of a proxy class, it performs
* the package access check on the interfaces it implements
* as in Class.getConstructor.
*
* If an interface is non-public, the proxy class must be defined by
* the defining loader of the interface. If the caller's class loader
* is not the same as the defining loader of the interface, the VM
* will throw IllegalAccessError when the generated proxy class is
* being defined via the defineClass0 method.
*/
private static void checkProxyAccess(Class<?> caller,
ClassLoader loader,
Class<?>... interfaces)
{
SecurityManager sm = System.getSecurityManager();
if (sm != null) {
ClassLoader ccl = caller.getClassLoader();
if (VM.isSystemDomainLoader(loader) && !VM.isSystemDomainLoader(ccl)) {
sm.checkPermission(SecurityConstants.GET_CLASSLOADER_PERMISSION);
}
ReflectUtil.checkProxyPackageAccess(ccl, interfaces);
}
}
/**
* Generate a proxy class. Must call the checkProxyAccess method
* to perform permission checks before calling this.
*/
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);
}
/*
* a key used for proxy class with 0 implemented interfaces
*/
private static final Object key0 = new Object();
/*
* Key1 and Key2 are optimized for the common use of dynamic proxies
* that implement 1 or 2 interfaces.
*/
/*
* a key used for proxy class with 1 implemented interface
*/
private static final class Key1 extends WeakReference<Class<?>> {
private final int hash;
Key1(Class<?> intf) {
super(intf);
this.hash = intf.hashCode();
}
@Override
public int hashCode() {
return hash;
}
@Override
public boolean equals(Object obj) {
Class<?> intf;
return this == obj ||
obj != null &&
obj.getClass() == Key1.class &&
(intf = get()) != null &&
intf == ((Key1) obj).get();
}
}
/*
* a key used for proxy class with 2 implemented interfaces
*/
private static final class Key2 extends WeakReference<Class<?>> {
private final int hash;
private final WeakReference<Class<?>> ref2;
Key2(Class<?> intf1, Class<?> intf2) {
super(intf1);
hash = 31 * intf1.hashCode() + intf2.hashCode();
ref2 = new WeakReference<Class<?>>(intf2);
}
@Override
public int hashCode() {
return hash;
}
@Override
public boolean equals(Object obj) {
Class<?> intf1, intf2;
return this == obj ||
obj != null &&
obj.getClass() == Key2.class &&
(intf1 = get()) != null &&
intf1 == ((Key2) obj).get() &&
(intf2 = ref2.get()) != null &&
intf2 == ((Key2) obj).ref2.get();
}
}
/*
* a key used for proxy class with any number of implemented interfaces
* (used here for 3 or more only)
*/
private static final class KeyX {
private final int hash;
private final WeakReference<Class<?>>[] refs;
@SuppressWarnings("unchecked")
KeyX(Class<?>[] interfaces) {
hash = Arrays.hashCode(interfaces);
refs = (WeakReference<Class<?>>[])new WeakReference<?>[interfaces.length];
for (int i = 0; i < interfaces.length; i++) {
refs[i] = new WeakReference<>(interfaces[i]);
}
}
@Override
public int hashCode() {
return hash;
}
@Override
public boolean equals(Object obj) {
return this == obj ||
obj != null &&
obj.getClass() == KeyX.class &&
equals(refs, ((KeyX) obj).refs);
}
private static boolean equals(WeakReference<Class<?>>[] refs1,
WeakReference<Class<?>>[] refs2) {
if (refs1.length != refs2.length) {
return false;
}
for (int i = 0; i < refs1.length; i++) {
Class<?> intf = refs1[i].get();
if (intf == null || intf != refs2[i].get()) {
return false;
}
}
return true;
}
}
/**
* A function that maps an array of interfaces to an optimal key where
* Class objects representing interfaces are weakly referenced.
*/
private static final class KeyFactory
implements BiFunction<ClassLoader, Class<?>[], Object>
{
@Override
public Object apply(ClassLoader classLoader, Class<?>[] interfaces) {
switch (interfaces.length) {
case 1: return new Key1(interfaces[0]); // the most frequent
case 2: return new Key2(interfaces[0], interfaces[1]);
case 0: return key0;
default: return new KeyX(interfaces);
}
}
}
/**
* A factory function that generates, defines and returns the proxy class given
* the ClassLoader and array of interfaces.
*/
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) {
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());
}
}
}
/**
* Returns an instance of a proxy class for the specified interfaces
* that dispatches method invocations to the specified invocation
* handler.
*
* <p>{@code Proxy.newProxyInstance} throws
* {@code IllegalArgumentException} for the same reasons that
* {@code Proxy.getProxyClass} does.
*
* @param loader the class loader to define the proxy class
* @param interfaces the list of interfaces for the proxy class
* to implement
* @param h the invocation handler to dispatch method invocations to
* @return a proxy instance with the specified invocation handler of a
* proxy class that is defined by the specified class loader
* and that implements the specified interfaces
* @throws IllegalArgumentException if any of the restrictions on the
* parameters that may be passed to {@code getProxyClass}
* are violated
* @throws SecurityException if a security manager, <em>s</em>, is present
* and any of the following conditions is met:
* <ul>
* <li> the given {@code loader} is {@code null} and
* the caller's class loader is not {@code null} and the
* invocation of {@link SecurityManager#checkPermission
* s.checkPermission} with
* {@code RuntimePermission("getClassLoader")} permission
* denies access;</li>
* <li> for each proxy interface, {@code intf},
* the caller's class loader is not the same as or an
* ancestor of the class loader for {@code intf} and
* invocation of {@link SecurityManager#checkPackageAccess
* s.checkPackageAccess()} denies access to {@code intf};</li>
* <li> any of the given proxy interfaces is non-public and the
* caller class is not in the same {@linkplain Package runtime package}
* as the non-public interface and the invocation of
* {@link SecurityManager#checkPermission s.checkPermission} with
* {@code ReflectPermission("newProxyInPackage.{package name}")}
* permission denies access.</li>
* </ul>
* @throws NullPointerException if the {@code interfaces} array
* argument or any of its elements are {@code null}, or
* if the invocation handler, {@code h}, is
* {@code null}
*/
@CallerSensitive
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);
}
}
private static void checkNewProxyPermission(Class<?> caller, Class<?> proxyClass) {
SecurityManager sm = System.getSecurityManager();
if (sm != null) {
if (ReflectUtil.isNonPublicProxyClass(proxyClass)) {
ClassLoader ccl = caller.getClassLoader();
ClassLoader pcl = proxyClass.getClassLoader();
// do permission check if the caller is in a different runtime package
// of the proxy class
int n = proxyClass.getName().lastIndexOf('.');
String pkg = (n == -1) ? "" : proxyClass.getName().substring(0, n);
n = caller.getName().lastIndexOf('.');
String callerPkg = (n == -1) ? "" : caller.getName().substring(0, n);
if (pcl != ccl || !pkg.equals(callerPkg)) {
sm.checkPermission(new ReflectPermission("newProxyInPackage." + pkg));
}
}
}
}
/**
* Returns true if and only if the specified class was dynamically
* generated to be a proxy class using the {@code getProxyClass}
* method or the {@code newProxyInstance} method.
*
* <p>The reliability of this method is important for the ability
* to use it to make security decisions, so its implementation should
* not just test if the class in question extends {@code Proxy}.
*
* @param cl the class to test
* @return {@code true} if the class is a proxy class and
* {@code false} otherwise
* @throws NullPointerException if {@code cl} is {@code null}
*/
public static boolean isProxyClass(Class<?> cl) {
return Proxy.class.isAssignableFrom(cl) && proxyClassCache.containsValue(cl);
}
/**
* Returns the invocation handler for the specified proxy instance.
*
* @param proxy the proxy instance to return the invocation handler for
* @return the invocation handler for the proxy instance
* @throws IllegalArgumentException if the argument is not a
* proxy instance
* @throws SecurityException if a security manager, <em>s</em>, is present
* and the caller's class loader is not the same as or an
* ancestor of the class loader for the invocation handler
* and invocation of {@link SecurityManager#checkPackageAccess
* s.checkPackageAccess()} denies access to the invocation
* handler's class.
*/
@CallerSensitive
public static InvocationHandler getInvocationHandler(Object proxy)
throws IllegalArgumentException
{
/*
* Verify that the object is actually a proxy instance.
*/
if (!isProxyClass(proxy.getClass())) {
throw new IllegalArgumentException("not a proxy instance");
}
final Proxy p = (Proxy) proxy;
final InvocationHandler ih = p.h;
if (System.getSecurityManager() != null) {
Class<?> ihClass = ih.getClass();
Class<?> caller = Reflection.getCallerClass();
if (ReflectUtil.needsPackageAccessCheck(caller.getClassLoader(),
ihClass.getClassLoader()))
{
ReflectUtil.checkPackageAccess(ihClass);
}
}
return ih;
}
private static native Class<?> defineClass0(ClassLoader loader, String name,
byte[] b, int off, int len);
}
其中，最重要的方法是
@CallerSensitive
public static Object newProxyInstance(ClassLoader loader,
Class<?>[] interfaces,
InvocationHandler h)
throws IllegalArgumentException
参数说明：
- ClassLoader loader
类的加载器 - Class<?>[] interfaces
得到全部的接口 - InvocationHandler h
得到InvocationHandler接口的子类的实例
- 抽象主题类
public interface Subject {
/**
* 接口方法
*/
public void request();
}
- 具体主题类
public class ConcreteSubject implements Subject {
/**
* 具体的业务逻辑实现
*/
@Override
public void request() {
//业务处理逻辑
}
}
- 动态创建代理对象的类
动态代理类只能代理接口（不支持抽象类），代理类都需要实现InvocationHandler类，实现invoke方法。invoke方法就是调用被代理接口的所有方法时需要调用的，返回的值是被代理接口的一个实现类。
package com.jerry.microservice.cloud.service.proxy.dynamic;
import lombok.extern.slf4j.Slf4j;
import java.lang.reflect.InvocationHandler;
import java.lang.reflect.Method;
import java.lang.reflect.Proxy;
/**
* @author :DuLvJL
* @date :23:07 2019/3/5
* @description :insert the description of this class
**/
@Slf4j
public class ProxyHandler implements InvocationHandler {
/**
* 目标对象
*/
private Object target;
/**
* 绑定关系，也就是关联到哪个接口（与具体的实现类绑定）的哪些方法将被调用时，执行invoke方法。
*
* @param target 绑定具体的代理实例
* @return 动态代理类实例
*/
public Object newProxyInstance(Object target) {
this.target = target;
/*
该方法用于为指定类装载器、一组接口及调用处理器生成动态代理类实例。
第一个参数指定产生代理对象的类加载器，需要将其指定为和目标对象同一个类加载器。
第二个参数要实现和目标对象一样的接口，所以只需要拿到目标对象的实现接口。
第三个参数表明这些被拦截的方法在被拦截时需要执行哪个InvocationHandler的invoke方法
根据传入的目标返回一个代理对象
*/
Object result = Proxy.newProxyInstance(target.getClass().getClassLoader(),
target.getClass().getInterfaces(), this);
return result;
}
/**
* 关联的这个实现类的方法被调用时将被执行。InvocationHandler接口的方法。
*
* @param proxy 代理
* @param method 原对象被调用的方法
* @param args 方法的参数
* @return
* @throws Throwable
*/
@Override
public Object invoke(Object proxy, Method method, Object[] args) throws Throwable {
//TODO 原对象方法调用前添加的预处理逻辑
Object ret = null;
try {
//调用目标方法
ret = method.invoke(target, args);
} catch (Exception e) {
log.error("调用{}.{}发生异常", target.getClass().getName(), method.getName(), e);
throw e;
}
//TODO 原对象方法调用后添加的后处理逻辑
return ret;
}
}
被代理对象target通过参数传递进来，我们通过target.getClass().getClassLoader()获取ClassLoader对象，然后通过target.getClass().getInterfaces()获取它实现的所有接口，然后将target包装到实现了InvocationHandler接口的ProxyHandler实现对象中。通过newProxyInstance函数我们就获得了一个动态代理对象。
- 客户端类
@Slf4j
public class Client {
public static void main(String[] args) {
log.info("开始");
ProxyHandler handler = new ProxyHandler();
Subject subject = (Subject) handler.newProxyInstance(new ConcreteSubject());
subject.request();
log.info("结束");
}
}
可以看到，我们可以通过ProxyHandler代理不同类型的对象，如果我们把对外的接口都通过动态代理来实现，那么所有的函数调用最终都会经过invoke函数的转发，因此我们就可以在这里做一些自己想做的操作，比如日志系统、事务、拦截器、权限控制等。
当前非常流行的面向切面的编程(Aspect Oriented Programming, AOP)，其核心就是动态代理机制。
备注：
要实现动态代理的首要条件：被代理类必须实现一个接口，才能被代理。
(现在还有CGLIB可以实现不用实现接口也可以实现动态代理。后续研究)
- 动态代理的优缺点
- 优点
动态代理与静态代理相比较，最大的好处是接口中声明的所有方法都被转移到调用处理器一个集中的方法中处理（InvocationHandler.invoke）。这样，在接口方法数量比较多的时候，我们可以进行灵活处理，而不需要像静态代理那样每一个方法进行中转。而且动态代理的应用使我们的类职责更加单一，复用性更强。 - 缺点
Proxy已经设计得非常优美，但是还是有一点点小小的遗憾之处——它始终无法摆脱仅支持 interface代理的桎梏，因为它的设计注定了这个遗憾。动态生成的代理类的继承关系图，已经注定有一个共同的父类叫 Proxy。Java 的继承机制注定了这些动态代理类们无法实现对 class 的动态代理，原因是多继承在 Java 中本质上就行不通。
8 总结
代理模式使用广泛，尤其是动态代理在大型开发框架中，应用较多并且高效。特别是使用AOP，实现代理模式就更方便了，比如使用Spring AOP和AspectJ这样的工具。
在使用AOP框架时，需要关注几个名词：
- 切面(Aspect)
- 切入点(JointPoint)
- 通知(Advice)
- 织入(Weave)
