JDK动态代理实现原理及手写JDK动态代理
1.JDK动态代理原理分析
- 在使用jdk动态代理的时候，必须要实现InvocationHandler接口，invoke方法Invoke 方法中该三个参数分别表示为: 代理对象、被代理执行的方法、参数
-
使用jdk动态代理获取代理类对象（JDK自动生成代理类） $Proxy0.class
-
将生成代理类加载至内存，让该类变成可执行类
2.源码分析
这里查看JDK1.8.0_191的源码，通过debug学习JDK动态代理的实现原理
首先我们当然是进入到Proxy的静态方法newProxyInstance中，代码附带注解如下：
@CallerSensitive
public static Object newProxyInstance(ClassLoader loader,
Class[] interfaces,
InvocationHandler h)
throws IllegalArgumentException
{
// 判断InvocationHandler是否为空，若为空，抛出空指针异常
Objects.requireNonNull(h);
//克隆接口的Class引用数组
final Class[] intfs = interfaces.clone();
final SecurityManager sm = System.getSecurityManager();
if (sm != null) {
checkProxyAccess(Reflection.getCallerClass(), loader, intfs);
}
/*
*生成接口的代理类的字节码文件
*/
Class cl = getProxyClass0(loader, intfs);
/*
*使用自定义的InvocationHandler作为参数，调用构造函数获取代理类对象实例
*/
try {
if (sm != null) {
checkNewProxyPermission(Reflection.getCallerClass(), cl);
}
//获取构造器
final Constructor cons = cl.getConstructor(constructorParams);
final InvocationHandler ih = h;
//若代理类不是public修饰,取消构造器的访问权限限制
if (!Modifier.isPublic(cl.getModifiers())) {
AccessController.doPrivileged(new PrivilegedAction() {
public Void run() {
cons.setAccessible(true);
return null;
}
});
}
//调用构造器来实例化一个代理对象
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);
}
} 这个方法还是挺简单的，就是先生成一个代理类的Class引用，然后通过调用Class引用中的构造方法来实例化一个代理对象，其中的关键的代码就是如何获取代理类一个Class引用，让我们进入方法getProxyClass0
private static Class getProxyClass0(ClassLoader loader,
Class... interfaces) {
//限定代理的接口不能超过65535个
if (interfaces.length > 65535) {
throw new IllegalArgumentException("interface limit exceeded");
}
// 若缓存中已经存在代理类的副本则直接返回,否则它将通过代理工厂类来创建代理类
return proxyClassCache.get(loader, interfaces);
}该方法中的proxyClassCache对象的声明如下所示，其中的ProxyClassFactory代码会在后面贴出来
/**
* a cache of proxy classes
*/
private static final WeakCache[], Class>
proxyClassCache = new WeakCache<>(new KeyFactory(), new ProxyClassFactory()); 通过代码查看proxyClassCache.get()方法：
public V get(K key, P parameter) {
Objects.requireNonNull(parameter);
expungeStaleEntries();
Object cacheKey = CacheKey.valueOf(key, refQueue);
//通过上面生成的key来获取对应的value,其中value是一个map存的是上面用来实例化proxyClassCache的KeyFactory和ProxyClassFactory
ConcurrentMap> valuesMap = map.get(cacheKey);
//若valueMap为null则初始化,若map中有旧的valueMap则赋值给valueMap
if (valuesMap == null) {
ConcurrentMap> oldValuesMap
= map.putIfAbsent(cacheKey,
valuesMap = new ConcurrentHashMap<>());
if (oldValuesMap != null) {
valuesMap = oldValuesMap;
}
}
//创建KeyFactory对象,并用此对象来获取提供代理类的对象
Object subKey = Objects.requireNonNull(subKeyFactory.apply(key, parameter));
Supplier supplier = valuesMap.get(subKey);
//提供代理类对象的工厂
Factory factory = null;
while (true) {
//若supplier不为null,则获取代理类
if (supplier != null) {
V value = supplier.get();
if (value != null) {
return value;
}
}
//若factory为null则实例化一个
if (factory == null) {
factory = new Factory(key, parameter, subKey, valuesMap);
}
//若supplier为空则赋值为factory并存到valueMap中
if (supplier == null) {
supplier = valuesMap.putIfAbsent(subKey, factory);
if (supplier == null) {
// successfully installed Factory
supplier = factory;
}
// else retry with winning supplier
} else {
//若替换成功则将supplier负责为factory,否则直接赋值为valueMap中存储的对象
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);
}
}
}
} 通过 V value = supplier.get()代码后可以看出关键的代码在与supplier对象的get方法和如何实例化出来的一个代理工厂类对象，让我们继续来看下Factory类的代码
private final class Factory implements Supplier {
private final K key;
private final P parameter;
private final Object subKey;
private final ConcurrentMap> valuesMap;
Factory(K key, P parameter, Object subKey,
ConcurrentMap> valuesMap) {
this.key = key;
this.parameter = parameter;
this.subKey = subKey;
this.valuesMap = valuesMap;
}
@Override
public synchronized V get() { // serialize access
// re-check
Supplier 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 {
value = Objects.requireNonNull(valueFactory.apply(key, parameter));
} finally {
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)
CacheValue 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;
}
} 通过value = Objects.requireNonNull(valueFactory.apply(key, parameter))代码中我们只需要关注get方法中的valueFactory对象的apply方法，该方法才是用来生成代理类的Class对象，代码如下：
private static final class ProxyClassFactory
implements BiFunction[], Class>
{
// 代理类的类名的前缀
private static final String proxyClassNamePrefix = "$Proxy";
// 代理类的编号，防止代理类名一致，通过原子类实现
private static final AtomicLong nextUniqueNumber = new AtomicLong();
@Override
public Class apply(ClassLoader loader, Class[] interfaces) {
Map, Boolean> interfaceSet = new IdentityHashMap<>(interfaces.length);
//遍历接口数组
for (Class intf : interfaces) {
//验证类加载器中是否存在该接口的一个Class对象
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");
}
//判断是否是接口
if (!interfaceClass.isInterface()) {
throw new IllegalArgumentException(
interfaceClass.getName() + " is not an interface");
}
//验证接口是否重复
if (interfaceSet.put(interfaceClass, Boolean.TRUE) != null) {
throw new IllegalArgumentException(
"repeated interface: " + interfaceClass.getName());
}
}
//代理类的包名
String proxyPkg = null;
int accessFlags = Modifier.PUBLIC | Modifier.FINAL;
//验证是否存在不是public修饰的接口在不同的包中
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) {
//如果没有非公共的接口,则包名使用com.sun.proxy
proxyPkg = ReflectUtil.PROXY_PACKAGE + ".";
}
//代理类的名称
long num = nextUniqueNumber.getAndIncrement();
String proxyName = proxyPkg + proxyClassNamePrefix + num;
//生成代理类的字节码文件
byte[] proxyClassFile = ProxyGenerator.generateProxyClass(
proxyName, interfaces, accessFlags);
try {
//通过字节码文件生成一个代理类的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());
}
}
} 上述代码中defineClass0是一个本地方法，可以将字节码文件生成我们需要的Class对象，而ProxyGenerator的generateProxyClass方法是用来生成字节码文件：
public static byte[] generateProxyClass(final String var0, Class[] var1, int var2) {
ProxyGenerator var3 = new ProxyGenerator(var0, var1, var2);
final byte[] var4 = var3.generateClassFile();
if (saveGeneratedFiles) {
AccessController.doPrivileged(new PrivilegedAction() {
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;
} 最终生成代理类：
public final class $Proxy0 extends Proxy implements OrderService {
private static Method m1;
private static Method m3;
private static Method m2;
private static Method m4;
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 void test() throws {
try {
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 void service() throws {
try {
super.h.invoke(this, m4, (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"));
m3 = Class.forName("com.dylan.proxy.service.OrderService").getMethod("test");
m2 = Class.forName("java.lang.Object").getMethod("toString");
m4 = Class.forName("com.dylan.proxy.service.OrderService").getMethod("service");
m0 = Class.forName("java.lang.Object").getMethod("hashCode");
} catch (NoSuchMethodException var2) {
throw new NoSuchMethodError(var2.getMessage());
} catch (ClassNotFoundException var3) {
throw new NoClassDefFoundError(var3.getMessage());
}
}
}可看到
1、代理类继承了Proxy类并且实现了要代理的接口，由于java不支持多继承，所以JDK动态代理不能代理类
2、重写了equals、hashCode、toString
3、有一个静态代码块，通过反射或者代理类的所有方法
4、通过invoke执行代理类中的目标方法doSomething
3.手写JDK动态代理
3.1JavaClassLoader
public class JavaClassLoader extends ClassLoader {
private File classPathFile;
public final static String CLASS_PATH = "/Users/wangqingfang/Desktop/ideaSpace/ext_proxy/src/main/java/com/dylan/ext/jdk/";
public JavaClassLoader() {
this.classPathFile = new File(CLASS_PATH);
}
/**
* 实现生成class文件并直接家在至内存
*
* @param name 文件名称该文件由extProxy动态生成
* @return
*/
@Override
public Class findClass(String name) {
String className = JdkProxy.PROXY_PACKAGE + "." + name;
if (classPathFile != null) {
File classFile = new File(classPathFile, name.replaceAll("\\.", "/") + ".class");
if (classFile.exists()) {
FileInputStream in = null;
ByteArrayOutputStream out = null;
try {
in = new FileInputStream(classFile);
out = new ByteArrayOutputStream();
byte[] buff = new byte[1024];
int len;
while ((len = in.read(buff)) != -1) {
out.write(buff, 0, len);
}
/**ps：该方法直接会将生成的class文件加载至内存*/
return defineClass(className+"", out.toByteArray(), 0, out.size());
} catch (Exception e) {
e.printStackTrace();
} finally {
if (in != null) {
try {
in.close();
} catch (IOException e) {
e.printStackTrace();
}
}
if (out != null) {
try {
out.close();
} catch (IOException e) {
e.printStackTrace();
}
}
}
}
}
return null;
}
}
3.2ExtJdkInvocationHandler接口
public interface ExtJdkInvocationHandler {
/**
* 动态代理对方法进行插入
*
* @param proxy 由jdk动态生成代理类@see JdkProxy.java
* @param method 通过代理需要调用的目标方法
* @param args 目标方法携带参数
* @return
* @throws Throwable
*/
Object invoke(Object proxy, Method method, Object[] args) throws Throwable;
}3.3JdkProxy
public class JdkProxy {
/**
* 代码换行
*/
private final static String RT = "\r\t";
/**
* 生成java文件位置
*/
public static final String PROXY_PACKAGE = "com.dylan.ext.jdk";
/**
* 生成class类名称
*/
private static final String proxyClassNamePrefix = "$Proxy";
/**
* 原子类 保证代理类的唯一性
*/
private static final AtomicLong nextUniqueNumber = new AtomicLong();
/**
* 动态生成代理类 暂不实现多个实现类情况
*
* @param loader
* @param classInfo
* @param h
* @return
* @throws IllegalArgumentException
*/
public static Object newProxyInstance(JavaClassLoader loader,
Class classInfo,
ExtJdkInvocationHandler h)
throws IllegalArgumentException {
String className = doClassPackage(classInfo);
javaCompilerClassInfo(className);
//4.使用classClassLoader 将$Proxy0.class读取到内存中...
Class classProxy = loader.findClass(className);
try {
//5.使用java反射机制给函数中赋值
Constructor m = classProxy.getConstructor(ExtJdkInvocationHandler.class);
Object o = m.newInstance(h);
return o;
} catch (Exception e) {
e.printStackTrace();
}
return null;
}
/**
* 对生成的java文件字符拼接
*
* @param classInfo
* @return
*/
public static String doClassPackage(Class classInfo) {
if (null == classInfo) {
throw new RuntimeException("Necessary parameter classInfo can`t be null");
}
Method[] methods = classInfo.getMethods();
String className = getClassNameInfo();
String proxyClass = "package " + PROXY_PACKAGE + ";" + RT
+ "import java.lang.reflect.Method;" + RT
+ "import java.lang.reflect.Method;" + RT
+ "import com.dylan.ext.jdk.handler.ExtJdkInvocationHandler;" + RT
+ "public class " + className + " implements " + classInfo.getName() + "{" + RT
+ "ExtJdkInvocationHandler h;" + RT
+ "public "+className+"(ExtJdkInvocationHandler h)" + "{" + RT
+ "this.h= h;" + RT + "}"
+ getMethodString(methods, classInfo) + RT + "}";
writeProxy(className, proxyClass);
return className;
}
/**
* 获取class类名称
*
* @return
*/
public static String getClassNameInfo() {
return proxyClassNamePrefix + nextUniqueNumber;
}
/**
* 生成代理方法
*
* @param methods
* @param classInfo
* @return
*/
public static String getMethodString(Method[] methods, Class classInfo) {
String proxyMe = "";
for (Method method : methods) {
Parameter[] parameters = method.getParameters();
proxyMe += "public void " + method.getName() + "() throws Throwable {" + RT
+ "Method md= " + classInfo.getName() + ".class.getMethod(\"" + method.getName()
+ "\",new Class[]{});" + RT
+ "this.h.invoke(this,md,null);" + RT + "}" + RT;
}
return proxyMe;
}
/**
* 将动态生成代理类写入到指定位置
*
* @param filename
* @param proxyClass
*/
public static void writeProxy(String filename, String proxyClass) {
/**此处应对filename proxyClass 进行合法性验证和非空验证*/
FileWriter fw = null;
try {
File f = new File(JavaClassLoader.CLASS_PATH + filename+".java");
fw = new FileWriter(f);
fw.write(proxyClass);
} catch (IOException e) {
e.printStackTrace();
} finally {
try {
fw.flush();
fw.close();
} catch (IOException e) {
e.printStackTrace();
}
}
}
/**
* 使用JavaJavaCompiler 编译该$Proxy0源代码 获取class文件
*
* @param filename
*/
public static void javaCompilerClassInfo(String filename) {
/**此处应对filename 进行合法性验证和非空验证*/
JavaCompiler compiler = ToolProvider.getSystemJavaCompiler();
StandardJavaFileManager fileMgr = null;
try {
fileMgr = compiler.getStandardFileManager(null, null, null);
} catch (Exception e) {
e.printStackTrace();
} finally {
try {
fileMgr.close();
} catch (IOException e) {
e.printStackTrace();
}
}
Iterable units = fileMgr.getJavaFileObjects(JavaClassLoader.CLASS_PATH + filename+".java");
JavaCompiler.CompilationTask t = compiler.getTask(null, fileMgr, null, null, null, units);
t.call();
}
}
3.4OrderInvocationHandler
public class OrderInvocationHandler implements ExtJdkInvocationHandler {
/**
* 目标代理对象
*/
public Object target;
public OrderInvocationHandler(Object target) {
this.target = target;
}
public OrderInvocationHandler() {
}
@Override
public Object invoke(Object proxy, Method method, Object[] args) throws Throwable {
before();
// 执行代理对象方法
Object result = method.invoke(target, args);
after();
return result;
}
/**
* 前置通知
*/
private void before() {//方法执行前
System.out.println("<<<<<<<<<<<<前置方法");
}
/**
* 后置通知
*/
private void after() {//方法执行后
System.out.println("<<<<<<<<<<<<后置方法");
}
/**
* 获取代理对象类以及将代理类加载到内存
*
* @param
* @return
*/
public T getProxy() {
return (T) JdkProxy.newProxyInstance(new JavaClassLoader(), this.target.getClass(), this);
}
public static void main(String[] args) throws Throwable{
OrderService orderService =(OrderService) JdkProxy.newProxyInstance(new JavaClassLoader(), OrderService.class, new OrderInvocationHandler(new OrderServiceImpl()));
orderService.service();
}
}
3.4OrderService及实现OrderServiceImpl
public interface OrderService {
/**
* 订单操作
*
* @throws Throwable
*/
void service() throws Throwable;
}public class OrderServiceImpl implements OrderService {
@Override
public void service() {
System.out.println("执行了订单操作。。。。。。。。");
}
}3.5运行后得到代理类
package com.dylan.ext.jdk;
import java.lang.reflect.Method;
import java.lang.reflect.Method;
import com.dylan.ext.jdk.handler.ExtJdkInvocationHandler;
public class $Proxy0 implements com.dylan.ext.jdk.service.OrderService {
ExtJdkInvocationHandler h;
public $Proxy0(ExtJdkInvocationHandler h) {
this.h = h;
}
public void service() throws Throwable {
Method md = com.dylan.ext.jdk.service.OrderService.class.getMethod("service", new Class[]{});
this.h.invoke(this, md, null);
}
}3.6最终运行结果
