特點
動态代理是AOP的核心技術，JDK的動态代理主要基于
java.lang.reflect.InvocationHandler
與
java.lang.reflect.Proxy
類，通過反射技術建立代理對象。
實作
建立一個
@Before
注解，在
Sales
接口上加上注解
public interface Sales {
@Before
void sale(Goods goods);
}
建立一個
InvocationHandler
，實作
invoke
方法
@Slf4j
public class ProxyHandler implements InvocationHandler {
private Sales target;
public ProxyHandler(Sales target) {
this.target = target;
}
@Override
public Object invoke(Object proxy, Method method, Object[] args) throws Throwable {
beforeInvoke(method);
Object invoke = method.invoke(this.target, args);
return invoke;
}
// 調用方法前的處理，當方法上能擷取到Before注解時，對原始對象再次代理，加入加價邏輯
private void beforeInvoke(Method method) {
Before annotation = method.getAnnotation(Before.class);
if (annotation != null){
this.target = new RestProxy(target);
}
}
}
測試代碼
@Test
public void testJDKProxy(){
Goods goods = new Goods();
Sales factory = new Factory();
ProxyHandler proxyHandler = new ProxyHandler(factory);
Object proxyInstance = Proxy.newProxyInstance(factory.getClass().getClassLoader(), new Class[]{Sales.class}, proxyHandler);
Sales proxy = (Sales) proxyInstance;
proxy.sale(goods);
}
日志資訊
2021-03-26 14:29:46.079 INFO 11576 --- [ main] org.example.proxy.rest.RestProxy : 商品加價前: Goods(super=[email protected], price=1200.00, name=手機)
2021-03-26 14:29:46.081 INFO 11576 --- [ main] org.example.proxy.rest.RestProxy : 商品加價後: Goods(super=[email protected], price=1700.00, name=手機)
2021-03-26 14:29:46.081 INFO 11576 --- [ main] org.example.proxy.rest.Factory : 商品最終價格: Goods(super=[email protected], price=1700.00, name=手機)
原理及源碼跟蹤
從
Proxy
類的方法進去檢視代碼：一通驗證之後，首先執行了
getProxyClass0
，然後擷取構造方法，并将構造方法設定為可通路，最後通過反射建立執行個體。
public static Object newProxyInstance(ClassLoader loader,
Class<?>[] interfaces,
InvocationHandler h)
throws IllegalArgumentException
{
// 驗證InvocationHandler不能為空
Objects.requireNonNull(h);
// 克隆複制接口
final Class<?>[] intfs = interfaces.clone();
// 擷取系統安全管理器
final SecurityManager sm = System.getSecurityManager();
if (sm != null) {
checkProxyAccess(Reflection.getCallerClass(), loader, intfs);
}
/*
* Look up or generate the designated proxy class.
*/
// 生成代理類
Class<?> cl = getProxyClass0(loader, intfs);
/*
* Invoke its constructor with the designated invocation handler.
*/
try {
if (sm != null) {
checkNewProxyPermission(Reflection.getCallerClass(), cl);
}
// 擷取構造方法
final Constructor<?> cons = cl.getConstructor(constructorParams);
final InvocationHandler ih = h;
// 判斷構造方法是不是public類型，若不是設定為允許操作
if (!Modifier.isPublic(cl.getModifiers())) {
AccessController.doPrivileged(new PrivilegedAction<Void>() {
public Void run() {
cons.setAccessible(true);
return null;
}
});
}
// 反射建立執行個體
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);
}
}
檢視生成代理類方法
getProxyClass0
可知，最終的類是從
proxyClassCache
緩存中擷取：
private static Class<?> getProxyClass0(ClassLoader loader,
Class<?>... interfaces) {
// 判斷接口數組的長度 不能大于 65535個
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
// 從緩存中擷取代理類
return proxyClassCache.get(loader, interfaces);
}
檢視
proxyClassCache
對象被聲明的代碼，建立了一個
WeakCache
，并傳遞進去了
KeyFactory
和
ProxyClassFactory
對象，這兩個類都是
Proxy
類的内部類，前者建立了代理類在緩存中的
key
，後者主要是代理類的生成過程：
/**
* a cache of proxy classes
*/
private static final WeakCache<ClassLoader, Class<?>[], Class<?>>
proxyClassCache = new WeakCache<>(new KeyFactory(), new ProxyClassFactory());
檢視
ProxyClassFactory
代碼，經過一系列的檢查，設定完成代理類名稱，然後調用
ProxyGenerator.generateProxyClass()
方法生成位元組碼，最後調用
defineClass0
方法生成類，
defineClass0
為
native
方法，主要檢視
ProxyGenerator.generateProxyClass()
方法；
/**
* 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
// 定義生成的代理類名稱字首
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");
}
}
}
// --------------------------------驗證參數結束-------------------------------------------
// 設定包名
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.
* 擷取生成代理類名稱=包名+字首+nextUniqueNumber.getAndIncrement()
*/
long num = nextUniqueNumber.getAndIncrement();
String proxyName = proxyPkg + proxyClassNamePrefix + num;
/*
* Generate the specified proxy class.
*/
// 生成位元組流 核心方法
byte[] proxyClassFile = ProxyGenerator.generateProxyClass(
proxyName, interfaces, accessFlags);
try {
// 調用native方法生成類
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());
}
}
}
檢視
ProxyGenerator
類中的
generateProxyClass
方法，該方法有3個參數，參數1是代理類名稱、參數2是接口類、參數3是
Modifier
類中16進制數字，該方法先是建立了一個
ProxyGenerator
對象，将3個參數通過構造方法初始化對象中屬性值，然後調用
generateClassFile()
方法生成位元組數組，最後通過
saveGeneratedFiles
屬性判斷是否需要生成代理檔案。
generateClassFile()
方法将
hashCode
,
equals
,
toString
都預設加入到代理類實作中；
public static byte[] generateProxyClass(final String var0, Class<?>[] var1, int var2) {
// 建立對象
ProxyGenerator var3 = new ProxyGenerator(var0, var1, var2);
// 生成位元組碼
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;
}
可以将屬性
saveGeneratedFiles
設定為
true
時，檢視最終生成的代理對象，
saveGeneratedFiles
屬性初始化
public static void main(String[] args) {
// 将saveGeneratedFiles置為true，運作
System.setProperty("sun.misc.ProxyGenerator.saveGeneratedFiles","true");
Goods goods = new Goods();
Sales factory = new Factory();
ProxyHandler proxyHandler = new ProxyHandler(factory);
Object proxyInstance = Proxy.newProxyInstance(factory.getClass().getClassLoader(), new Class[]{Sales.class}, proxyHandler);
Sales proxy = (Sales) proxyInstance;
proxy.sale(goods);
}
最後能看到的生成的代理位元組碼如下：
//
// Source code recreated from a .class file by IntelliJ IDEA
// (powered by Fernflower decompiler)
//
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 org.example.proxy.core.Goods;
import org.example.proxy.core.Sales;
// 類繼承了Proxy類，并實作了Sales接口
public final class $Proxy0 extends Proxy implements Sales {
private static Method m1;
private static Method m2;
private static Method m3;
private static Method m0;
// 構造函數 InvocationHandler 繼承父類的實作
public $Proxy0(InvocationHandler var1) throws {
super(var1);
}
// equals 方法實作
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);
}
}
//toString方法實作
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);
}
}
// 代理的方法實作 目前方法通過InvocationHandler對象執行了invoke方法
public final void sale(Goods var1) throws {
try {
super.h.invoke(this, m3, new Object[]{var1});
} catch (RuntimeException | Error var3) {
throw var3;
} catch (Throwable var4) {
throw new UndeclaredThrowableException(var4);
}
}
// hashCode 實作
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);
}
}
// 加載類和方法到jvm中
static {
try {
m1 = Class.forName("java.lang.Object").getMethod("equals", Class.forName("java.lang.Object"));
m2 = Class.forName("java.lang.Object").getMethod("toString");
m3 = Class.forName("org.example.proxy.core.Sales").getMethod("sale", Class.forName("org.example.proxy.core.Goods"));
m0 = Class.forName("java.lang.Object").getMethod("hashCode");
} catch (NoSuchMethodException var2) {
throw new NoSuchMethodError(var2.getMessage());
} catch (ClassNotFoundException var3) {
throw new NoClassDefFoundError(var3.getMessage());
}
}
}
至此，JDK動态代理源碼跟蹤完畢，不難總結出它的原理：
- 首先需要建立一個
InvocationHandler
invoke
- 通過
Proxy
newProxyInstance
ClassLoader
InvocationHandler
- 在建立代理對象的過程中，會通過
Proxy
getProxyClass0
Proxy
ProxyClassFactory
-
ProxyClassFactory
ProxyGenerator.generateProxyClass
- 最後通過
ProxyGenerator.defineClass0
JDK動态代理應用
是
Spring Aop
的底層技術，注解的底層實作、常用web攔截器等。
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