番外篇，jdk自带动态代理源码分析
之前和同学聊起代理模式，顺嘴提到了动态代理，就顺便看了一下源码，话不多说，开始分析，和之前一样为了方便理解，我会直接在代码中注释
这是一段很常见的动态代理代码，TestInterface是一个接口，里面只有一个test方法，TestInterfaceImpl类实现了TestInterface接口，代码也比较简单，我就不全部贴出来了
import java.lang.reflect.InvocationHandler;
import java.lang.reflect.Method;
import java.lang.reflect.Proxy;
public class Test implements InvocationHandler {
public TestInterface ti;
public Object newProxyInstance(TestInterface ti) {
this.ti = ti;
//重点
return Proxy.newProxyInstance(ti.getClass().getClassLoader(), ti.getClass().getInterfaces(), this);
}
@Override
public Object invoke(Object proxy, Method method, Object[] args) throws Throwable {
System.out.println("执行方法前的操作");
if(method.getName().equals("test"))
{
ti.test();
}
System.out.println("执行方法后的操作");
return null;
}
public static void main(String[] args) {
Test test =new Test ();
TestInterface ti = (TestInterface)test.newProxyInstance(new TestInterfaceImpl());
ti.test();
}
}
重点是Proxy.newProxyInstance方法
public static Object newProxyInstance(ClassLoader loader,
Class<?>[] interfaces,
InvocationHandler h) {
//若传入的动态代理类为空抛出异常
Objects.requireNonNull(h);
//获取当前系统的安全检查器
//因为这里会进行类加载和io操作，需要进行安全检查
final Class<?> caller = System.getSecurityManager() == null
? null
: Reflection.getCallerClass();
/*
* Look up or generate the designated proxy class and its constructor.
*/
//关键方法
Constructor<?> cons = getProxyConstructor(caller, loader, interfaces);
//此处只是调用Constructor的newInstance方法
//把动态代理对象传入，调用对应的构造方法
//返回一个动态代理后的实体类
//意义不大，这里就不多介绍
return newProxyInstance(caller, cons, h);
}
进入关键方法getProxyConstructor
private static Constructor<?> getProxyConstructor(Class<?> caller,
ClassLoader loader,
Class<?>... interfaces)
{
//判断需要代理的接口是否为多个，进行简单转换
//if else内容几乎一致，就只对前面的if内容进行分析，不再赘述
if (interfaces.length == 1) {
Class<?> intf = interfaces[0];
//判断是否需要进行安全检查
if (caller != null) {
checkProxyAccess(caller, loader, intf);
}
//重点方法，我会进行单独讲解
return proxyCache.sub(intf).computeIfAbsent(
loader,
(ld, clv) -> new ProxyBuilder(ld, clv.key()).build()
);
} else {
// interfaces cloned
final Class<?>[] intfsArray = interfaces.clone();
if (caller != null) {
checkProxyAccess(caller, loader, intfsArray);
}
final List<Class<?>> intfs = Arrays.asList(intfsArray);
return proxyCache.sub(intfs).computeIfAbsent(
loader,
(ld, clv) -> new ProxyBuilder(ld, clv.key()).build()
);
}
}
这段代码相当优雅，把lanbda表达式写的出神入化（但是我不建议大家平时这样写，可读性极差，我自己看这段代码蒙了有一会）
return proxyCache.sub(intf).computeIfAbsent(
loader,
(ld, clv) -> new ProxyBuilder(ld, clv.key()).build()
);
首先我们来看computeIfAbsent方法
public V computeIfAbsent(ClassLoader cl,
BiFunction<
? super ClassLoader,
? super CLV,
? extends V
> mappingFunction)
lanbda表达式推导的就是BiFunction接口的方法，进入BiFunction接口
public interface BiFunction<T, U, R> {
R apply(T t, U u);
。。。省略。。。
}
问题来了，我们可以把lanbda表达式写成下面的代码
R apply(T ld, U clv){
return new ProxyBuilder(ld, clv.key()).build();
}
然后通过类型推导（根据computeIfAbsent方法进行推导）将代码推导为
R apply(ClassLoader ld, U clv){
return new ProxyBuilder(ld, clv.key()).build();
}
再根据sub方法，和AbstractClassLoaderValue，Sub类进行进一步的推导
public abstract class AbstractClassLoaderValue<CLV extends AbstractClassLoaderValue<CLV, V>, V>
public final class Sub<K> extends AbstractClassLoaderValue<Sub<K>, V>
public <K> Sub<K> sub(K key) {
return new Sub<K>(key);
}
最终才能得到推导后的方法
R apply(ClassLoader ld, Sub clv){
return new ProxyBuilder(ld, clv.key()).build();
}
但是，此时返回值依然是不确定的，根据build的返回值推导，为Constructor，最终的方法才能得到
Constructor apply(ClassLoader ld, Sub clv){
return new ProxyBuilder(ld, clv.key()).build();
}
确实挺优雅的，就是太难理解，进入computeIfAbsent方法进行分析，设计的有点绕，猜测是为了解决复用性
public V computeIfAbsent(ClassLoader cl,
BiFunction<
? super ClassLoader,
? super CLV,
? extends V
> mappingFunction) throws IllegalStateException {
//创建map集合，注意，此处会根据类加载器加载对应的map集合
ConcurrentHashMap<CLV, Object> map = map(cl);
@SuppressWarnings("unchecked")
//获取自身对象此处为sub对象
CLV clv = (CLV) this;
Memoizer<CLV, V> mv = null;
while (true) {
//查看mv对象是否为空，如果为空(即，首次进入循环)，尝试从集合中获取vsl
//如果可以获取，认为该类已经加载，直接返回类对象
//否则进行添加并赋值给val
Object val = (mv == null) ? map.get(clv) : map.putIfAbsent(clv, mv);
//当第一次运行时，val和mv均为null
if (val == null) {
if (mv == null) {
// create Memoizer lazily when 1st needed and restart loop
//创建Memoizer对象
//只做一件事，将三个参数赋值给自身的成员变量
mv = new Memoizer<>(cl, clv, mappingFunction);
continue;
}
// mv != null, therefore sv == null was a result of successful
// putIfAbsent
try {
// trigger Memoizer to compute the value
//主要方法
V v = mv.get();
// attempt to replace our Memoizer with the value
//替换元素
map.replace(clv, mv, v);
// return computed value
return v;
} catch (Throwable t) {
// our Memoizer has thrown, attempt to remove it
map.remove(clv, mv);
// propagate exception because it's from our Memoizer
throw t;
}
} else {
try {
//如果进行到这一步，说明map集合中存在已经加载好的类
//判断是否为Memoizer（若为Memoizer可能是多线程条件下get方法尚未完成）
//然后返回对应的类，或是继续调用get方法
return extractValue(val);
} catch (Memoizer.RecursiveInvocationException e) {
// propagate recursive attempts to calculate the same
// value as being calculated at the moment
throw e;
} catch (Throwable t) {
// don't propagate exceptions thrown from foreign Memoizer -
// pretend that there was no entry and retry
// (foreign computeIfAbsent invocation will try to remove it anyway)
}
}
// TODO:
// Thread.onSpinLoop(); // when available
}
}
get方法
public V get() throws RecursiveInvocationException {
V v = this.v;
if (v != null) return v;
Throwable t = this.t;
if (t == null) {
synchronized (this) {
if ((v = this.v) == null && (t = this.t) == null) {
if (inCall) {
throw new RecursiveInvocationException();
}
inCall = true;
try {
this.v = v = Objects.requireNonNull(
//前面后面都是健壮性和复用性检测
//没什么好看的关键是是这里
//调用前面用lanbda表达式传入发方法
mappingFunction.apply(cl, clv));
} catch (Throwable x) {
this.t = t = x;
} finally {
inCall = false;
}
}
}
}
if (v != null) return v;
if (t instanceof Error) {
throw (Error) t;
} else if (t instanceof RuntimeException) {
throw (RuntimeException) t;
} else {
throw new UndeclaredThrowableException(t);
}
}
回到之前的build方法
Constructor<?> build() {
//核心，返回class对象
Class<?> proxyClass = defineProxyClass(module, interfaces);
final Constructor<?> cons;
try {
//返回对应的构造方法，没什么好看的
cons = proxyClass.getConstructor(constructorParams);
} catch (NoSuchMethodException e) {
throw new InternalError(e.toString(), e);
}
AccessController.doPrivileged(new PrivilegedAction<Void>() {
public Void run() {
cons.setAccessible(true);
return null;
}
});
return cons;
}
defineProxyClass方法
private static Class<?> defineProxyClass(Module m, List<Class<?>> interfaces) {
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; // non-public, final
String pkg = intf.getPackageName();
if (proxyPkg == null) {
proxyPkg = pkg;
} else if (!pkg.equals(proxyPkg)) {
throw new IllegalArgumentException(
"non-public interfaces from different packages");
}
}
}
if (proxyPkg == null) {
// all proxy interfaces are public
proxyPkg = m.isNamed() ? PROXY_PACKAGE_PREFIX + "." + m.getName()
: PROXY_PACKAGE_PREFIX;
} else if (proxyPkg.isEmpty() && m.isNamed()) {
throw new IllegalArgumentException(
"Unnamed package cannot be added to " + m);
}
if (m.isNamed()) {
if (!m.getDescriptor().packages().contains(proxyPkg)) {
throw new InternalError(proxyPkg + " not exist in " + m.getName());
}
}
/*
* Choose a name for the proxy class to generate.
*/
long num = nextUniqueNumber.getAndIncrement();
String proxyName = proxyPkg.isEmpty()
? proxyClassNamePrefix + num
: proxyPkg + "." + proxyClassNamePrefix + num;
ClassLoader loader = getLoader(m);
trace(proxyName, m, loader, interfaces);
/*
* Generate the specified proxy class.
*/
byte[] proxyClassFile = ProxyGenerator.generateProxyClass(
proxyName, interfaces.toArray(EMPTY_CLASS_ARRAY), accessFlags);
try {
Class<?> pc = UNSAFE.defineClass(proxyName, proxyClassFile,
0, proxyClassFile.length,
loader, null);
reverseProxyCache.sub(pc).putIfAbsent(loader, Boolean.TRUE);
return pc;
} 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());
}
}
其他都是做一些检验，没什么好说的，其实这么一大段重要的就下面
//生成动态代理字节码
byte[] proxyClassFile = ProxyGenerator.generateProxyClass(
proxyName, interfaces.toArray(EMPTY_CLASS_ARRAY), accessFlags);
try {
//通过类加载器加载字节码
Class<?> pc = UNSAFE.defineClass(proxyName, proxyClassFile,
0, proxyClassFile.length,
loader, null);
reverseProxyCache.sub(pc).putIfAbsent(loader, Boolean.TRUE);
return pc;
最终生成字节码也就是在内存中生成.class文件的方法，大家看看就行了，太多了，而且也没啥技术含量，方法和需要实现的接口方法内部字节码都是基本固定的，无非是把实现的接口方法转发到一个统一的方法中，再通过统一方法调用我们自己实现的invoke方法,就是实现的接口，导包的路径，类名称不同，稍有jvm常识就能自己写一个，就是费时间而已
private byte[] generateClassFile() {
/* ============================================================
* Step 1: Assemble ProxyMethod objects for all methods to
* generate proxy dispatching code for.
*/
/*
* Record that proxy methods are needed for the hashCode, equals,
* and toString methods of java.lang.Object. This is done before
* the methods from the proxy interfaces so that the methods from
* java.lang.Object take precedence over duplicate methods in the
* proxy interfaces.
*/
//添加hashCode，equals，toString方法
addProxyMethod(hashCodeMethod, Object.class);
addProxyMethod(equalsMethod, Object.class);
addProxyMethod(toStringMethod, Object.class);
/*
* Now record all of the methods from the proxy interfaces, giving
* earlier interfaces precedence over later ones with duplicate
* methods.
*/
for (Class<?> intf : interfaces) {
for (Method m : intf.getMethods()) {
addProxyMethod(m, intf);
}
}
/*
* For each set of proxy methods with the same signature,
* verify that the methods' return types are compatible.
*/
for (List<ProxyMethod> sigmethods : proxyMethods.values()) {
checkReturnTypes(sigmethods);
}
/* ============================================================
* Step 2: Assemble FieldInfo and MethodInfo structs for all of
* fields and methods in the class we are generating.
*/
try {
methods.add(generateConstructor());
//开始生成代理方法
for (List<ProxyMethod> sigmethods : proxyMethods.values()) {
for (ProxyMethod pm : sigmethods) {
// add static field for method's Method object
//生成对应的方法访问标志
fields.add(new FieldInfo(pm.methodFieldName,
"Ljava/lang/reflect/Method;",
ACC_PRIVATE | ACC_STATIC));
// generate code for proxy method and add it
//生成代理方法
methods.add(pm.generateMethod());
}
}
//添加静态块
methods.add(generateStaticInitializer());
} catch (IOException e) {
throw new InternalError("unexpected I/O Exception", e);
}
if (methods.size() > 65535) {
throw new IllegalArgumentException("method limit exceeded");
}
if (fields.size() > 65535) {
throw new IllegalArgumentException("field limit exceeded");
}
/* ============================================================
* Step 3: Write the final class file.
*/
/*
* Make sure that constant pool indexes are reserved for the
* following items before starting to write the final class file.
*/
//获取类名（稍后还要进行拼接）和父类名
cp.getClass(dotToSlash(className));
cp.getClass(superclassName);
//获取所有接口名
for (Class<?> intf: interfaces) {
cp.getClass(dotToSlash(intf.getName()));
}
/*
* Disallow new constant pool additions beyond this point, since
* we are about to write the final constant pool table.
*/
cp.setReadOnly();
ByteArrayOutputStream bout = new ByteArrayOutputStream();
DataOutputStream dout = new DataOutputStream(bout);
try {
/*
* Write all the items of the "ClassFile" structure.
* See JVMS section 4.1.
*/
// u4 magic;
//添加魔数
dout.writeInt(0xCAFEBABE);
// u2 minor_version;
//添加版本信息
dout.writeShort(CLASSFILE_MINOR_VERSION);
// u2 major_version;
//添加小版本信息
dout.writeShort(CLASSFILE_MAJOR_VERSION);
//添加常量池
cp.write(dout); // (write constant pool)
// u2 access_flags;
//添加标志位
dout.writeShort(accessFlags);
// u2 this_class;
//添加类名
dout.writeShort(cp.getClass(dotToSlash(className)));
// u2 super_class;
//添加父类名
dout.writeShort(cp.getClass(superclassName));
// u2 interfaces_count;
//添加接口表标志位
dout.writeShort(interfaces.length);
// u2 interfaces[interfaces_count];
//添加接口
for (Class<?> intf : interfaces) {
dout.writeShort(cp.getClass(
dotToSlash(intf.getName())));
}
// u2 fields_count;
//添加字段表标志位
dout.writeShort(fields.size());
// field_info fields[fields_count];
//添加字段
for (FieldInfo f : fields) {
f.write(dout);
}
// u2 methods_count;
//添加方法表标志位
dout.writeShort(methods.size());
// method_info methods[methods_count];
//添加方法
for (MethodInfo m : methods) {
m.write(dout);
}
// u2 attributes_count;
//类文件属性，稍后会在另外一个方法设置
dout.writeShort(0); // (no ClassFile attributes for proxy classes)
} catch (IOException e) {
throw new InternalError("unexpected I/O Exception", e);
}
return bout.toByteArray();
}
通过反射获取生成的class字节码
import java.io.FileOutputStream;
import java.io.IOException;
import java.lang.reflect.Method;
public class Test {
private static void saveProxyFile() {
FileOutputStream out = null;
try {
Class clazz = Class.forName("java.lang.reflect.ProxyGenerator");
Method m = clazz.getDeclaredMethod("generateProxyClass", String.class, Class[].class);
m.setAccessible(true);
byte[] bs = (byte[]) m.invoke(null, "$Proxy0", new Class[]{TestInterface.class});
out = new FileOutputStream("$Proxy0.class");
out.write(bs);
} catch (Exception e) {
e.printStackTrace();
} finally {
try {
if (out != null) {
out.flush();
out.close();
}
} catch (IOException e) {
e.printStackTrace();
}
}
}
public static void main(String[] args) {
saveProxyFile();
}
}
生成的class字节码
import java.lang.reflect.InvocationHandler;
import java.lang.reflect.Method;
import java.lang.reflect.Proxy;
import java.lang.reflect.UndeclaredThrowableException;
public final class $Proxy0 extends Proxy implements TestInterface {
//方法对象
private static Method m1;
private static Method m3;
private static Method m2;
private static Method m0;
//使用父类的构造方法处理
public $Proxy0(InvocationHandler var1) throws {
super(var1);
}
//将所有方法转发到父类h属性的invoke方法
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 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);
}
}
//将方法对象和实际方法关联
static {
try {
m1 = Class.forName("java.lang.Object").getMethod("equals", Class.forName("java.lang.Object"));
m3 = Class.forName("TestInterface").getMethod("test");
m2 = Class.forName("java.lang.Object").getMethod("toString");
m0 = Class.forName("java.lang.Object").getMethod("hashCode");
} catch (NoSuchMethodException var2) {
throw new NoSuchMethodError(var2.getMessage());
} catch (ClassNotFoundException var3) {
throw new NoClassDefFoundError(var3.getMessage());
}
}
}
父类的构造方法，最终所有方法都指向我们传入的类的invoke方法
protected Proxy(InvocationHandler h) {
Objects.requireNonNull(h);
this.h = h;
}
获取class对象的方法，因为是native 修饰，我就不继续下去了
public native Class<?> defineClass0(String name, byte[] b, int off, int len,
ClassLoader loader,
ProtectionDomain protectionDomain);
至此，分析结束