前言
最近學反射的時候了解了一下其應用——動态代理，覺得挺有意思，在此記錄一些對動态代理的了解，并對源碼進行簡單的梳理
反射
淺析動态代理之前先複習一下什麼是反射？
- 簡單了解反射就是以類對象作為入口，能夠獲得類内所有内容的一種方式
- 比如調用某個類中的方法時有兩種方式
- 常見的就是建立一個對象執行個體，然後調用其公共方法
- 另一種就是通過反射，先擷取類對象，通過類對象根據方法名擷取對應的方法，用invoke(obj,arg)的方式調用
- 在此就不進一步複習了，之是以先複習反射調用方法是因為了解動态代理要用
靜态代理
了解動态代理之前先了解靜态代理
直接丢例子吧
public static void staticProxyDemo() {
SuperMan superMan = new SuperMan();
superMan.say();
superMan.showMyName();
ProxySuperMan proxySuperMan = new ProxySuperMan();
proxySuperMan.say();
proxySuperMan.showMyName();
}
interface Human{
void showMyName();
void say();
}
class SuperMan implements Human{
@Override
public void showMyName() {
System.out.println("My name is SuperMan!");
}
@Override
public void say() {
System.out.println("Hello! I am SuperMan!");
}
}
class ProxySuperMan implements Human{
private SuperMan superMan = new SuperMan();
@Override
public void showMyName() {
System.out.println("我是proxy show");
superMan.showMyName();
}
@Override
public void say() {
System.out.println("我是proxy say");
superMan.say();
}
}
Hello! I am SuperMan!
My name is SuperMan!
我是proxy say
Hello! I am SuperMan!
我是proxy show
My name is SuperMan!
上述方式就是靜态代理，可以實作以下功能
- ProxySuperMan能夠提供和SuperMan一樣名字的方法調用
- 可以實作SuperMan的同名方法一樣的功能，并且還能在實作同名方法功能之前自定義一些方法（這就是最大的好處）
- 當我們需要在SuperMan的方法調用前後加一些功能時（在web項目中就比如驗證功能），就可以通過代理的方式添加，而不需去改原類SuperMan的源碼
動态代理
那麼為什麼要動态代理？
- 因為靜态代理方法名都是寫死的
- 如果要對多個類進行自定義意味着要自定義多個代理對象
- 比較麻煩
那麼我們希望動态代理能實作什麼功能呢？
- 我隻需要定義一個代理類
- 當我想代理某個類的某個方法時，我隻需要傳入該類名以及方法名即可
- 并且實作一個公共的代理邏輯，如在調用代理類方法前先執行某個方法
為此我自己寫了個簡單的動态代理方式去實作上述效果
public static void myProxyDemo() throws NoSuchMethodException, IllegalAccessException, InvocationTargetException {
MyProxy myProxyForSuperMan = new MyProxy(Human.class,new SuperMan());
myProxyForSuperMan.invoke("say",null);
}
class MyProxy{
private Class clazz;
private Object obj;
public MyProxy(Class clazz,Object obj) {
this.clazz=clazz;
this.obj=obj;
}
public void invoke(String methodName, Object[] args) throws NoSuchMethodException, InvocationTargetException, IllegalAccessException {
Method method = clazz.getMethod(methodName);
System.out.println("before invoke ......");
method.invoke(obj,args);
System.out.println("after invoke ......");
}
}
before invoke ......
Hello! I am SuperMan!
after invoke ......
- 隻需要在建立MyProxy傳入需要代理的接口以及一個執行個體對象
- 就能夠通過invoke傳入方法名和參數實作對應方法的代理
- 代理的邏輯隻需要自定義invoke即可
可以看到我自定義的方式雖然能夠完成動态代理的功能，但是看着很low，并且并沒有真正意義地建立一個代理類
那麼我們看看如果用官方的Proxy是怎麼進行代理的
public static void proxyDemo(){
HumanHandler humanHandler = new HumanHandler(new SuperMan());
Human proxy = (Human) Proxy.newProxyInstance(SuperMan.class.getClassLoader(), new Class[]{Human.class}, humanHandler);
proxy.showMyName();
proxy.say();
}
class HumanHandler implements InvocationHandler{
Object obj;
public HumanHandler(Object obj) {
this.obj = obj;
}
@Override
public Object invoke(Object proxy, Method method, Object[] args) throws Throwable {
System.out.println("before method: "+method.getName());
method.invoke(obj,args);
System.out.println("after method: "+method.getName());
return null;
}
}
before method: showMyName
My name is SuperMan!
after method: showMyName
before method: say
Hello! I am SuperMan!
after method: say
- 該方法能夠得到一個真正的代理類
- 并且該類能夠調用被代理類的所有同名方法
- 調用同名方法會執行在Handler中自定義的invoke邏輯
本文總結重點并不是怎麼用，而是搞清楚Proxy怎麼做到上述功能的
- 對比Proxy和我的方法，其實都是利用了invoke的方式傳入對象和方法進行調用
- 從本質上沒差別，代理的邏輯很簡單
我在看源碼前好奇的是，Proxy是怎麼做到能夠生成一個代理類并且該代理類擁有被代理類所有同名方法的，而且還能實作invoke邏輯而不是像我那樣隻能通過傳入名字來調，帶着這個問題我就去閱讀源碼
先進入 Proxy.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);
}
/*
* 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);
}
}
- 根據方法名很容易知道該方法做了什麼事情
-
cl = getProxyClass0
-
cl.getConstructor()
-
cons.newInstance(new Object[]{h})
我們接着看看怎麼獲得代理類的
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);
}
-
proxyClassCache.get()
- 如果緩存中有代理類，則直接傳回
- 沒有則建立
- 也就意味着Proxy對應同一組參數隻會生成的唯一一個代理類對象
那麼我們接着關注緩存中未命中怎麼建立代理類對象的，進入get方法
public V get(K key, P parameter) {
Objects.requireNonNull(parameter);
expungeStaleEntries();
Object cacheKey = CacheKey.valueOf(key, refQueue);
// lazily install the 2nd level valuesMap for the particular cacheKey
ConcurrentMap<Object, Supplier<V>> valuesMap = map.get(cacheKey);
if (valuesMap == null) {
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
Object subKey = Objects.requireNonNull(subKeyFactory.apply(key, parameter));
Supplier<V> supplier = valuesMap.get(subKey);
Factory factory = null;
while (true) {
if (supplier != null) {
// supplier might be a Factory or a CacheValue<V> instance
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
if (factory == null) {
factory = new Factory(key, parameter, subKey, valuesMap);
}
if (supplier == null) {
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);
}
}
}
}
- 隻需要關注未命中的情況，即
factory==null
- 此時會
factory = new Factory()
- 接着到
V value = supplier.get();
- 這裡的
supplier
factory
- 是以我們進入
Factory
get
public synchronized V get() { // serialize access
// re-check
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 {
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<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;
}
}
- 我們可以看到該方法還是被
synchronized
- 說明考慮到了線程安全，怎麼考慮的？現在先不考慮
- 擷取value值主要通過
value = Objects.requireNonNull(valueFactory.apply(key, parameter));
- 是以我們關注
valueFactory.apply(key, parameter)
- 由于
valueFactory
BiFunction<K, P, V>
- 是以我們去看傳了什麼給
valueFactory
- 可以看到
Factory
valueFactory
proxyClassCache
-
proxyClassCache
getProxyClass0
- 我們去看
proxyClassCache
private static final WeakCache<ClassLoader, Class<?>[], Class<?>>
proxyClassCache = new WeakCache<>(new KeyFactory(), new ProxyClassFactory());
- 通過
WeakCache
valueFactory
new ProxyClassFactory()
- 是以
valueFactory.apply(key, parameter)
ProxyClassFactory().apply()
- 我們接着看apply方法
@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());
}
}
}
- 代碼太多了
- 我們關注最核心的那就是
看到這裡其實還是沒有解決我閱讀源碼前提出的問題，即Proxy是怎麼生成一個能夠具備接口同名方法的代理類，并且調用invoke?
- 我想，答案應該就在
ProxyGenerator.generateProxyClass（）
- 是以我們接着看
為了更好地了解，我們從結果入手，即我們先試圖調用該方法看會得到什麼結果
private static void createProxyClassFile(){
String name = "ProxyHuman";
byte[] data = ProxyGenerator.generateProxyClass(name,new Class[]{Human.class});
FileOutputStream out =null;
try {
out = new FileOutputStream(name+".class");
System.out.println((new File("")).getAbsolutePath());
out.write(data);
} catch (FileNotFoundException e) {
e.printStackTrace();
} catch (IOException e) {
e.printStackTrace();
} finally {
if(null!=out) try {
out.close();
} catch (IOException e) {
e.printStackTrace();
}
}
}
H:\case_java8
- 我們在上述路徑中打開生成的class檔案
ProxyHuman.class
public final class ProxyHuman extends Proxy implements Human {
private static Method m1;
private static Method m2;
private static Method m4;
private static Method m3;
private static Method m0;
public ProxyHuman(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 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 showMyName() throws {
try {
super.h.invoke(this, m4, (Object[])null);
} catch (RuntimeException | Error var2) {
throw var2;
} catch (Throwable var3) {
throw new UndeclaredThrowableException(var3);
}
}
public final void say() throws {
try {
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"));
m2 = Class.forName("java.lang.Object").getMethod("toString");
m4 = Class.forName("hx.learn.Human").getMethod("showMyName");
m3 = Class.forName("hx.learn.Human").getMethod("say");
m0 = Class.forName("java.lang.Object").getMethod("hashCode");
} catch (NoSuchMethodException var2) {
throw new NoSuchMethodError(var2.getMessage());
} catch (ClassNotFoundException var3) {
throw new NoClassDefFoundError(var3.getMessage());
}
}
}
- 可以看到确實就是通過這個方法生成了一個代理類的位元組碼檔案
- 并且該class中除了接口的同名方法還有Object的方法
- 我們還可以看到每個方法都會調用invoke方法
super.h.invoke(this, m3, (Object[])null);
- 這意味着我們調用toString等Object方法時也會調用invoke
我們可以試試
public static void proxyDemo(){
HumanHandler humanHandler = new HumanHandler(new SuperMan());
Human proxy = (Human) Proxy.newProxyInstance(SuperMan.class.getClassLoader(), new Class[]{Human.class}, humanHandler);
// proxy.showMyName();
// proxy.say();
proxy.toString();
}
before method: toString
after method: toString
果然如此
那麼現在相當于隻是知道了代理類的結構，還沒能解決怎麼做到自動生成同名方法這一問題，我們需要去看
generateProxyClass（）
的實作
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<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;
}
- 我們可以看到主要方法在
generateClassFile()
由于該方法變量名很多而且沒有語義，閱讀比較困難。我們隻需要去找能夠解答疑惑的代碼段即可，下面我直接貼出我認為能夠解答疑惑的代碼段
我們要解決的兩個問題是
- 怎麼自動生成方法名
- 怎麼自動生成return并且調用invoke
怎麼自動生成方法名
- 先簡單說下主要邏輯
- 使用一個Map存放所有需要生成的方法名
- 周遊Map，把所有方法名write進位元組碼中
==================================
- 一開始先把三個Object的方法存入Map
this.addProxyMethod(hashCodeMethod, Object.class);
this.addProxyMethod(equalsMethod, Object.class);
this.addProxyMethod(toStringMethod, Object.class);
- 接着通過反射的
getMethods（）
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);
}
}
- 最後周遊Map進行
write
-
write
var15 = this.methods.iterator();
while(var15.hasNext()) {
ProxyGenerator.MethodInfo var21 = (ProxyGenerator.MethodInfo)var15.next();
var21.write(var14);
}
怎麼自動生成調用invoke的return代碼？
- 閱讀時我直接全文搜尋invoke
- 發現在
generateMethod()
而在
generateClassFile()
中調用了
generateMethod()
，并且最終也是加入Map
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());
}
}
至此，問題全部解決了
探索問題答案的過程還是比較有意思的
最後對流程做個簡單總結吧
- Proxy.newInstance會先檢視緩存是否有對應的代理類
- 命中則直接傳回，通過反射調用代理類構造方法建立執行個體對象傳回
- 不命中則建立
- 建立過程簡單的說就是用一個ProxyFactory模拟人寫代碼的過程
- 先檢視傳入的接口，看看被代理類有哪些方法
- 将這些方法名記錄下來
- 同時預設要把Object三個方法也記錄下來
- 并且還要記錄return應該寫什麼
- 全都記錄好以後調用write寫一個class出來就行了
最後的最後補充為什麼
Factory
的
get
要加鎖
- 因為get要操作緩存，緩存必須保證線程安全
下一步我打算去看看Spring的AOP源碼，再進行一次鞏固學習
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