文章目录
- 前言
- 概述
- 什么是代理
- 静态代理
- 动态代理
- 正文
- 入口
- 方法1：newProxyInstance
- 方法2：getProxyClass0
- 方法3：get 获取代理
- 方法4：apply 创建代理
- 方法5：generateProxyClass
- 方法6：generateClassFile
- 代理类
- 总结
前言
动态代理在开发可能使用的频率不是特别高，但在一些框架里面总能看到它的身影，例如Spring AOP就是使用JDK动态代理和CGLIB动态代理两种方法，本篇文章讲解JDK动态代理原理，看它是如何对类进行增强的。
概述
什么是代理
代理就是由直接调用转为间接调用，比如我们调用出单服务A，我需要在每次调用A时都记录日志，我们可以创建一个新的类及其方法B，在B中调用A，并且插入日志，这样由原先的调用A变成B，我们可以在B做一些增强操作如日志等。B就是代理类，而A就是原生类（被代理类）。
静态代理
静态代理的静态指的是代码运行前我们就编写了代理类，并且编译生成了.class文件，程序可以读取到内存并运行这些文件，如：
接口：
public interface Person {
public String work();
public String sleep();
}
代理类：
public class ProxyPerson implements Person {
private Person person;
public ProxyPerson(Person person) {
this.person = person;
}
@Override
public String work() {
System.out.println("增强");
return person.work();
}
@Override
public String sleep() {
System.out.println("增强");
return person.sleep();
}
}
通过代理类，我们可以创建Person接口实现类，传到ProxyPerson 完成代理。
动态代理
通过上述案例，可以看到静态代理的缺点，每增加一个不同的接口，都需要编写一个代理类，当业务代理多起来之后，难以维护。而动态代理就解决了这个问题，代理类是在运行过程中产生的，不需要给每一个被代理类创建代理对象，以JDK动态代理为例：
拦截处理器：
public class ProxyHandler implements InvocationHandler {
private Object target;
public Object getProxy(Object target){
this.target=target;
return Proxy.newProxyInstance(target.getClass().getClassLoader(), target.getClass().getInterfaces(), this);
}
@Override
public Object invoke(Object proxy, Method method, Object[] args) throws Throwable {
System.out.println("增强前");
Object invoke = method.invoke(target, args);
System.out.println("增强后");
return invoke;
}
}
接口：
public interface Person {
public String work();
public String sleep();
}
实现类：
public class Teacher implements Person {
@Override
public String work() {
String work="工作中";
System.out.println(work);
return work;
}
@Override
public String sleep() {
String sleep="睡觉中";
System.out.println(sleep);
return sleep;
}
}
测试用例：
public static void main(String[] args) {
Teacher teacher=new Teacher();
ProxyHandler proxyHandle=new ProxyHandler();
Person proxy = (Person) proxyHandle.getProxy(teacher);
proxy.sleep();
}
通过以上案例，可以知道我们并没有写死接口类型，无论是什么接口类型都传入原生类进行代理增强；
正文
既然我们知道动态代理概念，及其JDK动态代理的使用了，后面就来看看它的实现原理。
入口
以上述测试用例为入口，看一下Proxy.newProxyInstance是如何生成代理对象的；
public class ProxyHandler implements InvocationHandler {
private Object target;
public Object getProxy(Object target){
this.target=target;
return Proxy.newProxyInstance(target.getClass().getClassLoader(), target.getClass().getInterfaces(), this);
}
@Override
public Object invoke(Object proxy, Method method, Object[] args) throws Throwable {
System.out.println("增强前");
Object invoke = method.invoke(target, args);
System.out.println("增强后");
return invoke;
}
}
Proxy.newProxyInstance(target.getClass().getClassLoader(), target.getClass().getInterfaces(), this)，见方法1详解
方法1：newProxyInstance
public static Object newProxyInstance(ClassLoader loader,
Class<?>[] interfaces,
InvocationHandler h)
throws IllegalArgumentException
{
Objects.requireNonNull(h);
//拷贝一份，当前对象的接口集
final Class<?>[] intfs = interfaces.clone();
//获取安全管理，校验代理权限
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Class对象
Class<?> cl = getProxyClass0(loader, intfs);
/*
* Invoke its constructor with the designated invocation handler.
*/
try {
if (sm != null) {
checkNewProxyPermission(Reflection.getCallerClass(), cl);
}
//获取构造方法
final Constructor<?> cons = cl.getConstructor(constructorParams);
final InvocationHandler ih = h;
//如果不是公共修饰符
if (!Modifier.isPublic(cl.getModifiers())) {
AccessController.doPrivileged(new PrivilegedAction<Void>() {
public Void run() {
//设置权限
cons.setAccessible(true);
return null;
}
});
}
//实例化
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)，见方法2详解
方法2：getProxyClass0
private static Class<?> getProxyClass0(ClassLoader loader,
Class<?>... interfaces) {
//检验接口数量
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
//获取代理Clas对象
return proxyClassCache.get(loader, interfaces);
}
public V get(K key, P parameter) {
//校验校验不为空
Objects.requireNonNull(parameter);
expungeStaleEntries();
//将类加载器与ReferenceQueue队列封装成CacheKey作为缓存Key
Object cacheKey = CacheKey.valueOf(key, refQueue);
// lazily install the 2nd level valuesMap for the particular cacheKey
//尝试从缓存中获取之前处理过的缓存信息
ConcurrentMap<Object, Supplier<V>> valuesMap = map.get(cacheKey);
if (valuesMap == null) {
//如果不存在，则创建一个空容器并放到Map中
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
//根据策略选出接口并封装Key
Object subKey = Objects.requireNonNull(subKeyFactory.apply(key, parameter));
//获取当前函数式接口
Supplier<V> supplier = valuesMap.get(subKey);
Factory factory = null;
while (true) {
if (supplier != null) {
// supplier might be a Factory or a CacheValue<V> instance
//获取缓存后的factory，调用get方法创建代理对象
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
//创建Factory工厂
if (factory == null) {
factory = new Factory(key, parameter, subKey, valuesMap);
}
if (supplier == null) {
//将Factory存入valuesMap中
supplier = valuesMap.putIfAbsent(subKey, factory);
if (supplier == null) {
// successfully installed 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);
}
}
}
}
上面代码主要是为了加入缓存，创建代理逻辑在于调用Factory的get方法。
V value = supplier.get()，见方法3详解
方法3：get 获取代理
public synchronized V get() { // serialize access
// re-check
//校验
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 {
//创建代理对象入口
value = Objects.requireNonNull(valueFactory.apply(key, parameter));
} finally {
//没有创建成果，则移除缓存
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)
//新建缓存对象并存到reverseMap中
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;
}
}
Objects.requireNonNull(valueFactory.apply(key, parameter))，见方法4详解
方法4：apply 创建代理
public Class<?> apply(ClassLoader loader, Class<?>[] interfaces) {
Map<Class<?>, Boolean> interfaceSet = new IdentityHashMap<>(interfaces.length);
//遍历接口
for (Class<?> intf : interfaces) {
/*
* Verify that the class loader resolves the name of this
* interface to the same Class object.
*/
Class<?> interfaceClass = null;
//校验加载的接口是否一致
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.
*/
//校验是否是接口
if (!interfaceClass.isInterface()) {
throw new IllegalArgumentException(
interfaceClass.getName() + " is not an interface");
}
/*
* Verify that this interface is not a duplicate.
*/
//校验接口是否重复
if (interfaceSet.put(interfaceClass, Boolean.TRUE) != null) {
throw new IllegalArgumentException(
"repeated interface: " + interfaceClass.getName());
}
}
//包路径
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.
*/
//这里校验所在包修饰符是否是公共的，如果不是则获取所在包名，后续生成的代理对应需要在当前包下，这样才能引用被代理对象
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");
}
}
}
//如果所在包是公共的，则使用默认路径com.sun.proxy
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.
*/
//创建序号
long num = nextUniqueNumber.getAndIncrement();
//代理类名称
String proxyName = proxyPkg + proxyClassNamePrefix + num;
/*
* Generate the specified proxy class.
*/
//生成代理类流
byte[] proxyClassFile = ProxyGenerator.generateProxyClass(
proxyName, interfaces, accessFlags);
try {
//生成Class对象
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());
}
}
}
ProxyGenerator.generateProxyClass( proxyName, interfaces, accessFlags)，见方法5详解
方法5：generateProxyClass
public static byte[] generateProxyClass(final String var0, Class<?>[] var1, int var2) {
//var0：代理类名称 var1:接口 var2:包修饰符
ProxyGenerator var3 = new ProxyGenerator(var0, var1, var2);
//生成文件流
final byte[] var4 = var3.generateClassFile();
//这里判断是否要在本地生成代理类文件
if (saveGeneratedFiles) {
AccessController.doPrivileged(new PrivilegedAction<Void>() {
public Void run() {
//将“.”符号替换成系统路径符号，并将代理对象持久化
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;
}
var3.generateClassFile()，见方法6详解
方法6：generateClassFile
private byte[] generateClassFile() {
//添加Object对象的hashCode、equals、toString方法
this.addProxyMethod(hashCodeMethod, Object.class);
this.addProxyMethod(equalsMethod, Object.class);
this.addProxyMethod(toStringMethod, Object.class);
//获取接口集
Class[] var1 = this.interfaces;
//获取接口数量
int var2 = var1.length;
int var3;
Class var4;
//遍历接口
for(var3 = 0; var3 < var2; ++var3) {
//获取当前接口
var4 = var1[var3];
//获取当前接口的所有方法
Method[] var5 = var4.getMethods();
//当前接口方法的数量
int var6 = var5.length;
//遍历所有方法，添加到proxyMethods集合中
for(int var7 = 0; var7 < var6; ++var7) {
Method var8 = var5[var7];
this.addProxyMethod(var8, var4);
}
}
//获取代理方法迭代器
Iterator var11 = this.proxyMethods.values().iterator();
List var12;
//校验方法返回类型
while(var11.hasNext()) {
var12 = (List)var11.next();
checkReturnTypes(var12);
}
Iterator var15;
try {
//添加构造方法到methods中
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();
//添加方法名
this.fields.add(new ProxyGenerator.FieldInfo(var16.methodFieldName, "Ljava/lang/reflect/Method;", 10));
//添加处理后的MethodInfo对象信息
this.methods.add(var16.generateMethod());
}
}
//添加静态代码块
this.methods.add(this.generateStaticInitializer());
} catch (IOException var10) {
throw new InternalError("unexpected I/O Exception", var10);
}
//后面流程是生成代理对象过程
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);
}
}
}
代理类
通过上述方式，这时候会生成一个字节流的代理类，我们可以看到代理类同样实现了被代理类的接口，而且将其方法都解析成Method 对象，当调用其对应方法时，如work方法，会调用我们自定义的InvocationHandler 处理器的invoke方法，而invoke方法通过反射的方法去调用被代理类的work方法，在invoke方法中我们可以在调用被代理类前完成增强处理
public final class $Proxy0 extends Proxy implements Person {
private static Method m1;
private static Method m4;
private static Method m2;
private static Method m3;
private static Method m0;
public $Proxy0(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 String work() throws {
try {
return (String)super.h.invoke(this, m4, (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 String sleep() throws {
try {
return (String)super.h.invoke(this, m3, (Object[])null);
} catch (RuntimeException | Error var2) {
throw var2;
} catch (Throwable var3) {
throw new UndeclaredThrowableException(var3);
}
}
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"));
m4 = Class.forName("proxy.jdk.Person").getMethod("work");
m2 = Class.forName("java.lang.Object").getMethod("toString");
m3 = Class.forName("proxy.jdk.Person").getMethod("sleep");
m0 = Class.forName("java.lang.Object").getMethod("hashCode");
} catch (NoSuchMethodException var2) {
throw new NoSuchMethodError(var2.getMessage());
} catch (ClassNotFoundException var3) {
throw new NoClassDefFoundError(var3.getMessage());
}
}
}
总结
JDK动态代理的思路就是生成一个实现同个接口的代理类，代理类中拥有被代理类的所有方法，并且会在静态代码块中通过反射的放射解析拿到每个方法对应的Method 对象。代理类中的方法会将对应名称的Method 对象传递给我们自定义的InvocationHandler处理器，调用其invoke方法，而invoke方法中，由于有了Method 对象，我们可以通过反射的放射调用被代理类的方法，并且可以在被调用前增加逻辑处理，达到增强的效果。
通过源码的跟踪，我们知道了无论被代理类实现了什么接口，都可以对应的生成代理类，达到动态的效果。
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