搞懂Java動態代理
代理
代理模式的定義:為其他物件提供一種代理以控制對這個物件的訪問。即代理物件B控制真實物件A的訪問,那為什麼不直接使用真實物件?有時候真實物件不便直接使用或者不能直接使用,需要對真實物件進行增強,但又不能改變真實物件屬性(開閉原則),這時就需要用到代理類。
代理類主要負責為委託了(真實物件)預處理訊息、過濾訊息、傳遞訊息給委託類,代理類不現實具體服務,而是利用委託類來完成服務,並將執行結果封裝處理。有兩種方式的代理:
- 靜態代理:程式設計師將代理類預先建立好。
- 動態代理:代理類在程式執行過程中動態建立。
一個典型的代理模式通常有三個角色,這裡稱之為代理三要素:共同介面、真實物件、代理物件。
靜態代理
共同介面:
public interface Sell {
void add(String name);
void sell(String name);
}
真實物件:
public class Seller implements Sell {
@Override
public void add(String name) {
System.out.println("add product:" + name);
}
@Override
public void sell(String name) {
System.out.println("sell product:" + name);
}
}
代理物件:
public class ProxySeller implements Sell {
private Sell sell;
public ProxySeller(Sell sell){
this.sell = sell;
}
@Override
public void add(String name) {
sell.add(name);
}
@Override
public void sell(String name) {
sell.sell(name);
}
}
代理物件呼叫,真實物件Seller被傳遞給了代理物件ProxySeller,代理物件在執行具體方法時通過所持用的真實物件完成呼叫。
public static void main(String[] args) {
System.out.println("Hello World!");
Sell sell = new ProxySeller(new Seller());
sell.add("Apple");
sell.sell("Apple");
}
在處理部分物件增強功能時,靜態代理還是較合適,因為其直接落腳具體物件具體方法。但有時候面對不同物件、不同方法的代理場景時,這種代理模式就不是很靈活:
- 方式一:針對不同物件,分別建立一個代理類。
- 方式二:僅用一個代理類,實現n個不同的介面,最終分類呼叫。
在方案一中,會重複建立邏輯相同僅引用物件不同的代理類;方案二中會造成一個代理類的無線膨脹,最終難以控制及維護。接下來,引出動態代理…
動態代理
共同介面、真實物件不變,我們構建一個代理物件呼叫處理程式InvocationHandler。
import java.lang.reflect.InvocationHandler;
import java.lang.reflect.Method;
public class SellerInvocationHandler implements InvocationHandler {
private Object delegate;
public SellerInvocationHandler(Object delegate){
this.delegate = delegate;
}
@Override
public Object invoke(Object proxy, Method method, Object[] args) throws Throwable {
System.out.println("Befor");
method.invoke(delegate,args);
System.out.println("After");
return null;
}
}
執行動態代理:
public static void main(String[] args) {
System.out.println("Hello World!");
Sell delegate = new Seller();
InvocationHandler handler = new SellerInvocationHandler(delegate);
Sell proxy = (Sell)Proxy.newProxyInstance(delegate.getClass().getClassLoader(),
delegate.getClass().getInterfaces(),
handler);
proxy.add("Apple");
proxy.sell("Apple");
}
通過Proxy類的靜態方法newProxyInstance返回一個介面的代理例項。針對不同的代理類,傳入相應的代理程式控制器InvocationHandler。如果新來一個委託類PrimaryStudent,如:
public interface Student {
void buy(String name);
void say(String name);
}
public class PrimaryStudent implements Student {
@Override
public void buy(String name) {
System.out.println("Student Buy " + name);
}
@Override
public void say(String name) {
System.out.println("Student Say " + name);
}
}
實現對這個類的動態代理過程如下:
public static void main(String[] args) {
System.out.println("Hello World!");
Sell delegate = new Seller();
Student student = new PrimaryStudent();
InvocationHandler handler = new SellerInvocationHandler(delegate);
InvocationHandler handler1 = new SellerInvocationHandler(student);
Sell proxy = (Sell)Proxy.newProxyInstance(delegate.getClass().getClassLoader(),
delegate.getClass().getInterfaces(),
handler);
Student proxy1 = (Student)Proxy.newProxyInstance(student.getClass().getClassLoader(),
student.getClass().getInterfaces(),
handler1);
proxy.add("Apple");
proxy.sell("Apple");
proxy1.say("Apple");
proxy1.buy("Apple");
}
其實動態代理的基本要素仍然是:1、共同介面;2、真是物件;3代理物件(只不過此時的代理類是自動生成的)。Spring AOP就是動態代理的典型實現。
道理差不多明白了,接下來繼續瞭解動態代理底層的過程,及代理類如何生成的…
底層過程
newProxyInstance方法究竟是如何生成一個代理例項的,通過其原始碼展開說明:
@CallerSensitive
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);
}
/*
* 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;
}
});
}
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);
}
}
整體流程:
- 1、生成代理類Proxy的Class物件。
- 2、如果Class作用域為私有,通過 setAccessible 支援訪問
- 3、獲取Proxy Class建構函式,建立Proxy代理例項。
其中,利用getProxyClass0(loader, intfs)生成代理類Proxy的Class物件,其原始碼如下,其中有說明,如果指定介面的代理類已經存在與快取中,則不用新建立,直接從快取中取即可;如果快取中沒有指定代理物件,則通過ProxyClassFactory來建立一個代理物件。
/**
* Generate a proxy class. Must call the checkProxyAccess method
* to perform permission checks before calling this.
*/
private static Class<?> getProxyClass0(ClassLoader loader,
Class<?>... interfaces) {
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
return proxyClassCache.get(loader, interfaces);
}
ProxyClassFactory內部類建立、定義代理類,返回給定ClassLoader 和interfaces的代理類。其中核心方法就是apply,
/**
* 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");
}
/*
* 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();
String proxyName = proxyPkg + proxyClassNamePrefix + num;
/*
* 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());
}
}
}
proxyClassFile是生產的代理類位元組碼,generateProxyClass是真正生成代理類class位元組碼的函式,進去ProxyGenerator類的靜態方法generateProxyClass。
public static byte[] generateProxyClass(final String name,
Class[] interfaces)
{
ProxyGenerator gen = new ProxyGenerator(name, interfaces);
// 這裡動態生成代理類的位元組碼,由於比較複雜就不進去看了
final byte[] classFile = gen.generateClassFile();
// 如果saveGeneratedFiles的值為true,則會把所生成的代理類的位元組碼儲存到硬碟上
if (saveGeneratedFiles) {
java.security.AccessController.doPrivileged(
new java.security.PrivilegedAction<Void>() {
public Void run() {
try {
FileOutputStream file =
new FileOutputStream(dotToSlash(name) + ".class");
file.write(classFile);
file.close();
return null;
} catch (IOException e) {
throw new InternalError(
"I/O exception saving generated file: " + e);
}
}
});
}
// 返回代理類的位元組碼
return classFile;
}
位元組碼生成後,呼叫defineClass0來解析位元組碼,生成了Proxy的Class物件。在瞭解完代理類動態生成過程後,生產的代理類是怎樣的,誰來執行這個代理類。
ProxyGenerator.generateProxyClass函式中注意下面一點:
if(saveGeneratedFiles) {
...
FileOutputStream file = new FileOutputStream(dotToSlash(name) + ".class");
file.write(classFile);
...
}
saveGeneratedFiles定義如下,其指代是否儲存生成的代理類class檔案,預設false不儲存。
private static final boolean saveGeneratedFiles = ((Boolean)AccessController.doPrivileged(new GetBooleanAction("sun.misc.ProxyGenerator.saveGeneratedFiles"))).booleanValue();
但是為了加深理解,我們可以在main函式中修改此係統變數,main中定義了2個真實物件Sell及Student,接下來看看其生成的代理物件。
public static void main(String[] args) {
System.getProperties().setProperty("sun.misc.ProxyGenerator.saveGeneratedFiles", "true");
Sell delegate = new Seller();
Student student = new PrimaryStudent();
InvocationHandler handler = new SellerInvocationHandler(delegate);
InvocationHandler handler1 = new SellerInvocationHandler(student);
Sell proxy = (Sell)Proxy.newProxyInstance(delegate.getClass().getClassLoader(),
delegate.getClass().getInterfaces(),
handler);
Student proxy1 = (Student)Proxy.newProxyInstance(student.getClass().getClassLoader(),
student.getClass().getInterfaces(),
handler1);
proxy.add("Apple");
proxy.sell("Apple");
proxy1.say("Apple");
proxy1.buy("Apple");
}
如圖,生成了兩個名為 $Proxy0.class、$Proxy1.class的class檔案,利用idea的反編譯能力,開啟兩檔案,其內容如下:
package com.sun.proxy;
import java.lang.reflect.InvocationHandler;
import java.lang.reflect.Method;
import java.lang.reflect.Proxy;
import java.lang.reflect.UndeclaredThrowableException;
public final class $Proxy0 extends Proxy implements Sell {
private static Method m1;
private static Method m2;
private static Method m4;
private static Method m3;
private static Method m0;
//這個是$Proxy0繼承Proxy並呼叫了父類的構造方法
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})).booleanValue();
} catch (RuntimeException | Error var3) {
throw var3;
} catch (Throwable var4) {
throw new UndeclaredThrowableException(var4);
}
}
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);
}
}
//實現介面Sell的代理方法sell
public final void sell(String var1) throws {
try {
super.h.invoke(this, m4, new Object[]{var1});
} catch (RuntimeException | Error var3) {
throw var3;
} catch (Throwable var4) {
throw new UndeclaredThrowableException(var4);
}
}
//實現介面Sell的代理方法add
public final void add(String var1) throws {
try {
super.h.invoke(this, m3, 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)).intValue();
} 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"));
m2 = Class.forName("java.lang.Object").getMethod("toString");
m4 = Class.forName("Sell").getMethod("sell", Class.forName("java.lang.String"));
m3 = Class.forName("Sell").getMethod("add", Class.forName("java.lang.String"));
m0 = Class.forName("java.lang.Object").getMethod("hashCode");
} catch (NoSuchMethodException var2) {
throw new NoSuchMethodError(var2.getMessage());
} catch (ClassNotFoundException var3) {
throw new NoClassDefFoundError(var3.getMessage());
}
}
}
$Proxy0繼承了Proxy類,實現了Sell介面,是Sell物件的代理類。生成的代理類呼叫委託類方法時,呼叫InvocationHandler的invoke方法。
總結
其實,動態代理實現思路與靜態代理一樣,也是“共同介面、真實物件、代理物件”,只不過靜態代理的代理物件是程式設計師預先寫好的,而動態代理的代理物件是利用位元組碼手段動態生成代理物件的,減少了重複勞動,所以優秀的程式設計師可能也是一個“懶惰”的程式設計師。