1 代理模式的定義
代理模式：代理模式又叫委托模式读恃，是為某個對象提供一個代理對象招盲，并且由代理對象控制對原對象的訪問。代理模式通俗來講就是我們生活中常見的中介纺涤。
代理模式可以提供非常好的訪問控制桦锄，應(yīng)用比較廣泛。
舉個例子來說明：假如說我現(xiàn)在想買一輛二手車蔫耽，雖然我可以自己去找車源结耀，做質(zhì)量檢測等一系列的車輛過戶流程留夜，但是這確實(shí)太浪費(fèi)我得時間和精力了。我只是想買一輛車而已為什么我還要額外做這么多事呢图甜？于是我就通過中介公司來買車碍粥，他們來給我找車源，幫我辦理車輛過戶流程黑毅，我只是負(fù)責(zé)選擇自己喜歡的車嚼摩，然后付錢就可以了。
代理模式的通用類圖：
代理模式的通用類圖
- Subject
抽象主題角色：可以是抽象類矿瘦，也可以是接口枕面。抽象主題是一個普通的業(yè)務(wù)類型，無特殊要求缚去。 - RealSubject
具體主題角色：也叫做被委托角色或被代理角色潮秘，是業(yè)務(wù)邏輯的具體執(zhí)行者。 - Proxy
代理主題角色：也叫做委托類或代理類易结。它負(fù)責(zé)對真實(shí)角色的應(yīng)用枕荞，把所有抽象主題類定義的方法限制委托給真實(shí)主題角色實(shí)現(xiàn)，并且在具體主題角色處理完畢前后做預(yù)處理和善后處理工作搞动。
一個代理類可以代理多個被委托者或被代理者躏精，因此一個代理類具體代理哪個具體主題角色，是由場景類決定的鹦肿。最簡單的情況是一個主題類和一個代理類矗烛。通常情況下，一個接口只需要一個代理類狮惜，具體代理哪個實(shí)現(xiàn)類有高層模塊決定高诺。
2 代理模式的優(yōu)點(diǎn)
職責(zé)清晰
具體角色是實(shí)現(xiàn)具體的業(yè)務(wù)邏輯，不用關(guān)心其他非本職責(zé)的事務(wù)碾篡，通過后期的代理完成一件事務(wù)虱而，代碼清晰。在某些情況下开泽，一個客戶類不想或者不能直接引用一個委托對象牡拇，而代理類對象可以在客戶類和委托對象之間起到中介的作用，其特征是代理類和委托類實(shí)現(xiàn)相同的接口穆律。高擴(kuò)展性
具體主題角色隨時會發(fā)生變化吵瞻，但是只要實(shí)現(xiàn)了接口丈氓，接口不變，代理類就可以不做任何修改繼續(xù)使用，符合“開閉原則”齐邦。
另外，代理類除了是客戶類和委托類的中介之外浪感，我們還可以通過給代理類增加額外的功能來擴(kuò)展委托類的功能，這樣做我們只需要修改代理類而不需要再修改委托類少梁，同樣符合開閉原則。智能化待完善
動態(tài)代理的智能化
3 代理模式的使用場景
代理類主要負(fù)責(zé)為委托類預(yù)處理消息矫付、過濾消息凯沪、把消息轉(zhuǎn)發(fā)給委托類，以及事后對返回結(jié)果的處理等买优。代理類本身并不真正實(shí)現(xiàn)服務(wù)妨马，而是通過調(diào)用委托類的相關(guān)方法，來提供特定的服務(wù)杀赢。真正的業(yè)務(wù)功能還是由委托類來實(shí)現(xiàn)烘跺，但是可以在業(yè)務(wù)功能執(zhí)行的前后加入一些公共的服務(wù)。例如我們想給項(xiàng)目加入緩存葵陵、日志這些功能液荸，我們就可以使用代理類來完成，而沒必要打開已經(jīng)封裝好的委托類脱篙。
4 代理模式通用示例
為了保持行為的一致性娇钱，代理類和委托類通常會實(shí)現(xiàn)相同的接口，所以在訪問者看來兩者沒有絲毫的區(qū)別绊困。通過代理類這中間一層文搂，能有效控制對委托類對象的直接訪問，也可以很好地隱藏和保護(hù)委托類對象秤朗，同時也為實(shí)施不同控制策略預(yù)留了空間煤蹭，從而在設(shè)計上獲得了更大的靈活性。
更通俗的說取视，代理解決的問題是：當(dāng)兩個類需要通信時硝皂，引入第三方代理類，將兩個類的關(guān)系解耦作谭，讓我們只了解代理類即可稽物，而且代理的出現(xiàn)還可以讓我們完成與另一個類之間的關(guān)系的統(tǒng)一管理。但是切記折欠，代理類和委托類要實(shí)現(xiàn)相同的接口贝或，因?yàn)榇碚嬲{(diào)用的還是委托類的方法。
代理模式有多種不同的實(shí)現(xiàn)方式锐秦。如果按照代理創(chuàng)建的時期來進(jìn)行分類：靜態(tài)代理咪奖、動態(tài)代理。
- 靜態(tài)代理
由程序員創(chuàng)建或特定工具自動生成源代碼酱床，再對其進(jìn)行編譯羊赵。在程序運(yùn)行之前，代理類.class文件就已經(jīng)被創(chuàng)建扇谣，代理類和委托類的關(guān)系在運(yùn)行前就確定昧捷。 - 動態(tài)代理
動態(tài)代理類的源碼是在程序運(yùn)行期間由JVM根據(jù)反射等機(jī)制動態(tài)的生成揖闸，所以不存在代理類的字節(jié)碼文件。代理類和委托類的關(guān)系是在程序運(yùn)行時確定料身。
4.1 靜態(tài)代理
- 抽象主題類
public interface Subject {
/**
* 接口方法
*/
public void request();
}
- 具體主題類
public class ConcreteSubject implements Subject {
/**
* 具體的業(yè)務(wù)邏輯實(shí)現(xiàn)
*/
@Override
public void request() {
//業(yè)務(wù)處理邏輯
}
}
- 代理類
public class Proxy implements Subject {
/**
* 要代理的實(shí)現(xiàn)類
*/
private Subject subject = null;
/**
* 默認(rèn)代理自己
*/
public Proxy() {
this.subject = new Proxy();
}
public Proxy(Subject subject) {
this.subject = subject;
}
/**
* 構(gòu)造函數(shù)，傳遞委托者
*
* @param objects 委托者
*/
public Proxy(Object... objects) {
}
/**
* 實(shí)現(xiàn)接口方法
*/
@Override
public void request() {
this.before();
this.subject.request();
this.after();
}
/**
* 預(yù)處理
*/
private void before() {
//do something
}
/**
* 后處理
*/
private void after() {
//do something
}
}
- 客戶端類
public class Client {
public static void main(String[] args) {
Subject subject = new ConcreteSubject();
Proxy proxy = new Proxy(subject);
proxy.request();
}
}
- 靜態(tài)代理優(yōu)缺點(diǎn)
- 優(yōu)點(diǎn)
業(yè)務(wù)類只需要關(guān)注業(yè)務(wù)邏輯本身衩茸，保證了業(yè)務(wù)類的重用性芹血。這是代理的共有優(yōu)點(diǎn)。代理使客戶端不需要知道實(shí)現(xiàn)類是什么楞慈，怎么做的幔烛，而客戶端只需知道代理即可（解耦合）。對于如上的客戶端代碼囊蓝，Subject subject = new ConcreteSubject(); Proxy proxy = new Proxy(subject);可以應(yīng)用工廠方法將它隱藏饿悬。 - 缺點(diǎn)
- 代理類和委托類實(shí)現(xiàn)了相同的接口，代理類通過委托類實(shí)現(xiàn)了相同的方法聚霜。這樣就出現(xiàn)了大量的代碼重復(fù)狡恬。如果接口增加一個方法，除了所有實(shí)現(xiàn)類需要實(shí)現(xiàn)這個方法外蝎宇，所有代理類也需要實(shí)現(xiàn)此方法弟劲。增加了代碼維護(hù)的復(fù)雜度。
- 代理對象只服務(wù)于一種類型的對象姥芥，如果要服務(wù)多類型的對象兔乞。勢必要為每一種對象都進(jìn)行代理，靜態(tài)代理在程序規(guī)模稍大時就無法勝任了凉唐。如上的代碼是只為Subject類的訪問提供了代理庸追，但是如果還要為其他類如AnotherSubject類提供代理的話，就需要我們再次添加代理AnotherSubject的代理類台囱。
由于靜態(tài)代理的這個缺點(diǎn)淡溯，就需要使用動態(tài)代理。
4.2 動態(tài)代理
從靜態(tài)代理會發(fā)現(xiàn)——每個代理類只能為一個接口服務(wù)玄坦，這樣程序開發(fā)中必然會產(chǎn)生許多的代理類血筑。所以我們想辦法通過一個代理類完成全部的代理功能，那么我們就需要用動態(tài)代理.
在上面4.1的示例中煎楣，一個代理只能代理一種類型豺总，而且是在編譯器就已經(jīng)確定被代理的對象。而動態(tài)代理是在運(yùn)行時择懂，通過反射機(jī)制實(shí)現(xiàn)動態(tài)代理喻喳，并且能夠代理各種類型的對象。
在Java中要想實(shí)現(xiàn)動態(tài)代理機(jī)制困曙，需要java.lang.reflect.InvocationHandler接口和 java.lang.reflect.Proxy類的支持表伦。
-
java.lang.reflect.InvocationHandler接口的定義
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;
}
參數(shù)解釋：
- Object proxy
被代理對象 - Method method
要調(diào)用的方法 - Object[] args
方法調(diào)用時所需要的參數(shù)
-
java.lang.reflect.Proxy類的定義
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);
}
其中谦去，最重要的方法是
@CallerSensitive
public static Object newProxyInstance(ClassLoader loader,
Class<?>[] interfaces,
InvocationHandler h)
throws IllegalArgumentException
參數(shù)說明：
- ClassLoader loader
類的加載器 - Class<?>[] interfaces
得到全部的接口 - InvocationHandler h
得到InvocationHandler接口的子類的實(shí)例
- 抽象主題類
public interface Subject {
/**
* 接口方法
*/
public void request();
}
- 具體主題類
public class ConcreteSubject implements Subject {
/**
* 具體的業(yè)務(wù)邏輯實(shí)現(xiàn)
*/
@Override
public void request() {
//業(yè)務(wù)處理邏輯
}
}
- 動態(tài)創(chuàng)建代理對象的類
動態(tài)代理類只能代理接口（不支持抽象類），代理類都需要實(shí)現(xiàn)InvocationHandler類蹦哼，實(shí)現(xiàn)invoke方法鳄哭。invoke方法就是調(diào)用被代理接口的所有方法時需要調(diào)用的，返回的值是被代理接口的一個實(shí)現(xiàn)類纲熏。
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 {
/**
* 目標(biāo)對象
*/
private Object target;
/**
* 綁定關(guān)系妆丘，也就是關(guān)聯(lián)到哪個接口（與具體的實(shí)現(xiàn)類綁定）的哪些方法將被調(diào)用時，執(zhí)行invoke方法局劲。
*
* @param target 綁定具體的代理實(shí)例
* @return 動態(tài)代理類實(shí)例
*/
public Object newProxyInstance(Object target) {
this.target = target;
/*
該方法用于為指定類裝載器勺拣、一組接口及調(diào)用處理器生成動態(tài)代理類實(shí)例。
第一個參數(shù)指定產(chǎn)生代理對象的類加載器鱼填，需要將其指定為和目標(biāo)對象同一個類加載器药有。
第二個參數(shù)要實(shí)現(xiàn)和目標(biāo)對象一樣的接口，所以只需要拿到目標(biāo)對象的實(shí)現(xiàn)接口苹丸。
第三個參數(shù)表明這些被攔截的方法在被攔截時需要執(zhí)行哪個InvocationHandler的invoke方法
根據(jù)傳入的目標(biāo)返回一個代理對象
*/
Object result = Proxy.newProxyInstance(target.getClass().getClassLoader(),
target.getClass().getInterfaces(), this);
return result;
}
/**
* 關(guān)聯(lián)的這個實(shí)現(xiàn)類的方法被調(diào)用時將被執(zhí)行愤惰。InvocationHandler接口的方法。
*
* @param proxy 代理
* @param method 原對象被調(diào)用的方法
* @param args 方法的參數(shù)
* @return
* @throws Throwable
*/
@Override
public Object invoke(Object proxy, Method method, Object[] args) throws Throwable {
//TODO 原對象方法調(diào)用前添加的預(yù)處理邏輯
Object ret = null;
try {
//調(diào)用目標(biāo)方法
ret = method.invoke(target, args);
} catch (Exception e) {
log.error("調(diào)用{}.{}發(fā)生異常", target.getClass().getName(), method.getName(), e);
throw e;
}
//TODO 原對象方法調(diào)用后添加的后處理邏輯
return ret;
}
}
被代理對象target通過參數(shù)傳遞進(jìn)來赘理，我們通過target.getClass().getClassLoader()獲取ClassLoader對象羊苟，然后通過target.getClass().getInterfaces()獲取它實(shí)現(xiàn)的所有接口，然后將target包裝到實(shí)現(xiàn)了InvocationHandler接口的ProxyHandler實(shí)現(xiàn)對象中感憾。通過newProxyInstance函數(shù)我們就獲得了一個動態(tài)代理對象蜡励。
- 客戶端類
@Slf4j
public class Client {
public static void main(String[] args) {
log.info("開始");
ProxyHandler handler = new ProxyHandler();
Subject subject = (Subject) handler.newProxyInstance(new ConcreteSubject());
subject.request();
log.info("結(jié)束");
}
}
可以看到，我們可以通過ProxyHandler代理不同類型的對象阻桅，如果我們把對外的接口都通過動態(tài)代理來實(shí)現(xiàn)凉倚，那么所有的函數(shù)調(diào)用最終都會經(jīng)過invoke函數(shù)的轉(zhuǎn)發(fā)，因此我們就可以在這里做一些自己想做的操作嫂沉，比如日志系統(tǒng)稽寒、事務(wù)、攔截器趟章、權(quán)限控制等杏糙。
當(dāng)前非常流行的面向切面的編程(Aspect Oriented Programming, AOP)，其核心就是動態(tài)代理機(jī)制蚓土。
備注：
要實(shí)現(xiàn)動態(tài)代理的首要條件：被代理類必須實(shí)現(xiàn)一個接口宏侍，才能被代理。
(現(xiàn)在還有CGLIB可以實(shí)現(xiàn)不用實(shí)現(xiàn)接口也可以實(shí)現(xiàn)動態(tài)代理蜀漆。后續(xù)研究)
- 動態(tài)代理的優(yōu)缺點(diǎn)
- 優(yōu)點(diǎn)
動態(tài)代理與靜態(tài)代理相比較谅河，最大的好處是接口中聲明的所有方法都被轉(zhuǎn)移到調(diào)用處理器一個集中的方法中處理（InvocationHandler.invoke）。這樣，在接口方法數(shù)量比較多的時候绷耍，我們可以進(jìn)行靈活處理吐限，而不需要像靜態(tài)代理那樣每一個方法進(jìn)行中轉(zhuǎn)。而且動態(tài)代理的應(yīng)用使我們的類職責(zé)更加單一褂始，復(fù)用性更強(qiáng)诸典。 - 缺點(diǎn)
Proxy已經(jīng)設(shè)計得非常優(yōu)美，但是還是有一點(diǎn)點(diǎn)小小的遺憾之處——它始終無法擺脫僅支持 interface代理的桎梏崎苗，因?yàn)樗脑O(shè)計注定了這個遺憾搂赋。動態(tài)生成的代理類的繼承關(guān)系圖，已經(jīng)注定有一個共同的父類叫 Proxy益缠。Java 的繼承機(jī)制注定了這些動態(tài)代理類們無法實(shí)現(xiàn)對 class 的動態(tài)代理，原因是多繼承在 Java 中本質(zhì)上就行不通基公。
8 總結(jié)
代理模式使用廣泛幅慌，尤其是動態(tài)代理在大型開發(fā)框架中，應(yīng)用較多并且高效轰豆。特別是使用AOP胰伍，實(shí)現(xiàn)代理模式就更方便了，比如使用Spring AOP和AspectJ這樣的工具酸休。
在使用AOP框架時骂租，需要關(guān)注幾個名詞：
- 切面(Aspect)
- 切入點(diǎn)(JointPoint)
- 通知(Advice)
- 織入(Weave)
