說到動態(tài)代理的受限于的接口的,這包括兩個方面
- 非接口不能使用JDK動態(tài)代理
- 若一個類實現(xiàn)了接口(implements), 其非接口定義的method也不能使用JDK動態(tài)代理
為何Cglib可以代理普通類,而JDK 動態(tài)代理的受限于接口的原因是什么?
先說結論,這是因為JDK 動態(tài)代理會生成一個新的類Proxy$N,這個類會直接繼承java.lang.reflect.Proxy,起JDK源碼為
package java.lang.reflect;
public class Proxy implements java.io.Serializable {
....
}
那么根據(jù)Java類的單繼承語法,所有的類只能是單繼承的,所以Proxy$N和被代理的對象的完全兩個不同的實現(xiàn)類,壓根不可能存在繼承關系(代理對象不可能繼承代理的對象).
但是這能解釋JDK動態(tài)代理受限于接口么?
顯然不可以,我們知道靜態(tài)代理的代理對象和被代理的對象就沒有接口限制這一說.
那么真的原因需要看源碼.
我寫了一個例子
public class JdkProxyTest {
class ProxyHandler implements InvocationHandler {
// 被代理的對象
private Object target;
public ProxyHandler(Object target) {
this.target = target;
}
@Override
public Object invoke(Object proxy, Method method, Object[] args) throws Throwable {
return method.invoke(target, args);
}
}
@Test
void interfaceTest() {
//predicate是個匿名類的實例(Predicate是一個接口,這里用到lambda語法)
Predicate<Integer> predicate = it -> it > 100;
ProxyHandler handler = new ProxyHandler(predicate);
ClassLoader classLoader = predicate.getClass().getClassLoader();
Predicate proxy = (Predicate) Proxy.newProxyInstance(classLoader,
new Class[]{Predicate.class}, handler);
System.out.println(predicate.getClass() + "\t" + proxy.getClass());
System.out.println("result: \t" + proxy.test(50) + "\t" + proxy.test(500));
}
}
Proxy的方法 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);
}
/*
* 很關鍵的一個方法,求代理對象的類型
* 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;
}
});
}
//反射生成對象
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);
}
}
通過源碼可以知道代理對象的是下面這行生成的
Class<?> cl = getProxyClass0(loader, intfs);
繼續(xù)看源碼
/**
* a cache of proxy classes
*/
private static final WeakCache<ClassLoader, Class<?>[], Class<?>>
proxyClassCache = new WeakCache<>(new KeyFactory(), new ProxyClassFactory());
private static Class<?> getProxyClass0(ClassLoader loader,
Class<?>... interfaces) {
if (interfaces.length > 65535) {
throw new IllegalArgumentException("interface limit exceeded");
}
// 代理的對象類通過緩存起來
return proxyClassCache.get(loader, interfaces);
}
看到這還是沒看到代理的類是生成邏輯? Debug跟吧
image.png
line 124 是個循環(huán)
line 142 - 145 若cacha中沒有生成代理類的supplier那么就將factory賦值給supplier并且從line142看到supplier放入到了cache中,下面看supplier(可以看到是WeakCache$Factory)的實現(xiàn)
我們看到com.lang.relect.Proxy$ProxyClassFactory的apply()方法實現(xiàn)了代理類
/**
* 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");
}
/*
* 若不是接口類型 拋出異常
* 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();
//代理類的名稱
String proxyName = proxyPkg + proxyClassNamePrefix + num;
/*
* 生成代理類的二進制,也就是生成代理類的核心邏輯
* 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());
}
}
}
apply方法實現(xiàn)了
- 若不是接口類型,拋出異常, 這兒從代碼層面印證JDK動態(tài)代理受限于接口
- 生成代理類的名稱
- 生成代理類(class)的二進制流
byte[] proxyClassFile = ProxyGenerator.generateProxyClass(
proxyName, interfaces, accessFlags);
public static byte[] generateProxyClass(final String var0, Class<?>[] var1, int var2) {
ProxyGenerator var3 = new ProxyGenerator(var0, var1, var2);
final byte[] var4 = var3.generateClassFile();
// 是否保存成文件中
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;
}
這個方法主要是
- 調(diào)用方法實現(xiàn)代理類class文件生成
- 保存class文件
private static final boolean saveGeneratedFiles = (Boolean)AccessController.doPrivileged(new GetBooleanAction("sun.misc.ProxyGenerator.saveGeneratedFiles"));
若需要保存文件,在添加JVM參數(shù): -Dsun.misc.ProxyGenerator.saveGeneratedFiles=true
image.png
生成的文件格式如下:
image.png
//
// 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 java.util.function.Predicate;
public final class $Proxy9 extends Proxy implements Predicate {
private static Method m1;
private static Method m3;
private static Method m2;
private static Method m6;
private static Method m4;
private static Method m5;
private static Method m7;
private static Method m0;
public $Proxy9(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 Predicate negate() throws {
try {
return (Predicate)super.h.invoke(this, m3, (Object[])null);
} catch (RuntimeException | Error var2) {
throw var2;
} catch (Throwable var3) {
throw new UndeclaredThrowableException(var3);
}
}
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 Predicate and(Predicate var1) throws {
try {
return (Predicate)super.h.invoke(this, m6, new Object[]{var1});
} catch (RuntimeException | Error var3) {
throw var3;
} catch (Throwable var4) {
throw new UndeclaredThrowableException(var4);
}
}
public final Predicate isEqual(Object var1) throws {
try {
return (Predicate)super.h.invoke(this, m4, new Object[]{var1});
} catch (RuntimeException | Error var3) {
throw var3;
} catch (Throwable var4) {
throw new UndeclaredThrowableException(var4);
}
}
public final boolean test(Object var1) throws {
try {
return (Boolean)super.h.invoke(this, m5, new Object[]{var1});
} catch (RuntimeException | Error var3) {
throw var3;
} catch (Throwable var4) {
throw new UndeclaredThrowableException(var4);
}
}
public final Predicate or(Predicate var1) throws {
try {
return (Predicate)super.h.invoke(this, m7, new Object[]{var1});
} catch (RuntimeException | Error var3) {
throw var3;
} catch (Throwable var4) {
throw new UndeclaredThrowableException(var4);
}
}
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"));
m3 = Class.forName("java.util.function.Predicate").getMethod("negate");
m2 = Class.forName("java.lang.Object").getMethod("toString");
m6 = Class.forName("java.util.function.Predicate").getMethod("and", Class.forName("java.util.function.Predicate"));
m4 = Class.forName("java.util.function.Predicate").getMethod("isEqual", Class.forName("java.lang.Object"));
m5 = Class.forName("java.util.function.Predicate").getMethod("test", Class.forName("java.lang.Object"));
m7 = Class.forName("java.util.function.Predicate").getMethod("or", Class.forName("java.util.function.Predicate"));
m0 = Class.forName("java.lang.Object").getMethod("hashCode");
} catch (NoSuchMethodException var2) {
throw new NoSuchMethodError(var2.getMessage());
} catch (ClassNotFoundException var3) {
throw new NoClassDefFoundError(var3.getMessage());
}
}
}
可以清晰的看到代理為 com.sun.proxy.$Proxy9,繼承了java.lang.reflect.Proxy,實現(xiàn)了
java.util.function.Predicate接口,并且所有的方法的實現(xiàn)均掉用super.h屬性(InvocationHandler)這個對象就Proxy.newProxyInstance()的是否傳遞進來的(看Proxy源碼確實有屬性InvocationHandler h)
那么就清楚了所謂的動態(tài)的最終的實現(xiàn)思想生成若干的靜態(tài)代理類(思想
歸根溯源還是靜態(tài)代理)
下面是com.sun.proxy.$Proxy9實現(xiàn)細節(jié) 還是sum.misc.ProxyGenerator類
private byte[] generateClassFile() {
this.addProxyMethod(hashCodeMethod, Object.class);
this.addProxyMethod(equalsMethod, Object.class);
this.addProxyMethod(toStringMethod, Object.class);
Class[] var1 = this.interfaces;
int var2 = var1.length;
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);
}
}
Iterator var11 = this.proxyMethods.values().iterator();
List var12;
while(var11.hasNext()) {
var12 = (List)var11.next();
checkReturnTypes(var12);
}
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());
}
}
this.methods.add(this.generateStaticInitializer());
} catch (IOException var10) {
throw new InternalError("unexpected I/O Exception", var10);
}
if (this.methods.size() > 65535) {
throw new IllegalArgumentException("method limit exceeded");
} else if (this.fields.size() > 65535) {
throw new IllegalArgumentException("field limit exceeded");
} else {
this.cp.getClass(dotToSlash(this.className));
this.cp.getClass("java/lang/reflect/Proxy");
var1 = this.interfaces;
var2 = var1.length;
for(var3 = 0; var3 < var2; ++var3) {
var4 = var1[var3];
this.cp.getClass(dotToSlash(var4.getName()));
}
this.cp.setReadOnly();
ByteArrayOutputStream var13 = new ByteArrayOutputStream();
DataOutputStream var14 = new DataOutputStream(var13);
try {
var14.writeInt(-889275714);
var14.writeShort(0);
var14.writeShort(49);
this.cp.write(var14);
var14.writeShort(this.accessFlags);
var14.writeShort(this.cp.getClass(dotToSlash(this.className)));
var14.writeShort(this.cp.getClass("java/lang/reflect/Proxy"));
var14.writeShort(this.interfaces.length);
Class[] var17 = this.interfaces;
int var18 = var17.length;
for(int var19 = 0; var19 < var18; ++var19) {
Class var22 = var17[var19];
var14.writeShort(this.cp.getClass(dotToSlash(var22.getName())));
}
var14.writeShort(this.fields.size());
var15 = this.fields.iterator();
while(var15.hasNext()) {
ProxyGenerator.FieldInfo var20 = (ProxyGenerator.FieldInfo)var15.next();
var20.write(var14);
}
var14.writeShort(this.methods.size());
var15 = this.methods.iterator();
while(var15.hasNext()) {
ProxyGenerator.MethodInfo var21 = (ProxyGenerator.MethodInfo)var15.next();
var21.write(var14);
}
var14.writeShort(0);
return var13.toByteArray();
} catch (IOException var9) {
throw new InternalError("unexpected I/O Exception", var9);
}
}
}
回到上面的問題, 為什么靜態(tài)代理不受限與接口而JDK 動態(tài)代理要首先呢?
是因為JDK 在實現(xiàn)的時候需哪些方法?是根據(jù)傳遞接口確定的,所以代理類是不會有被代理類非接口(hascode, euqals等除外,因為這些是object基本方法,任何對象都會有),上面說了Java單繼承的語法也是一個原因
總之: JDK 動態(tài)代理的實現(xiàn)方案和Java單繼承的語法導致了這個限制的存在.
還有個問題嗎,動態(tài)代理類(Class)每次都要重新生成一遍嗎?
不是的:
再次獲取動態(tài)代理的的時候,supplier變成了WeakChache$CacheValue(本文上面說過首次生成的時候supplier是WeakCache$Factory),直接返回,不會重新生成一遍
image.png
那是什么時候放進去的呢?
image.png
WeakCache.Factory的get()方法在
line240-243代理類的緩存邏輯
