Java 8 Lambda實現原理分析！

為了支持函數式編程，Java?8引入了Lambda表達式，那么在Java?8中到底是如何實現Lambda表達式的呢??Lambda表達式經過編譯之后，到底會生成什么東西呢??在沒有深入分析前，讓我們先想一想，Java?8中每一個Lambda表達式必須有一個函數式接口與之對應。

那么你或許在想Lambda表達式是不是轉化成與之對應的函數式接口的一個實現類呢，然后通過多態(tài)的方式調用子類的實現呢，如下面代碼是一個Lambda表達式的樣例：

@FunctionalInterface
interface?Print<T>?{
????public?void?print(T?x);
}
public?class?Lambda?{???
????public?static?void?PrintString(String?s,?Print?print)?{
????????print.print(s);
????}
????public?static?void?main(String\[\]?args)?{
????????PrintString("test",?(x)?->?System.out.println(x));
????}
}

按照上面的分析，理論上經過編譯器處理后，最終生成的代碼應該如下面所示：

@FunctionalInterface
interface?Print<T>?{
????public?void?print(T?x);
}

class?Lambda$$0?implements?Print<String>?{
????@Override
????public?void?print(String?x)?{
????????System.out.println(x);
????}
}

public?class?Lambda?{???
????public?static?void?PrintString(String?s,?
????????????Print?print)?{
????????print.print(s);
????}
????public?static?void?main(String\[\]?args)?{
????????PrintString("test",?new?Lambda$$0());
????}
}

再或者是一個內部類實現，代碼如下所示：

@FunctionalInterface
interface?Print<T>?{
????public?void?print(T?x);
}
public?class?Lambda?{???
????final?class?Lambda$$0?implements?Print<String>?{
????????@Override
????????public?void?print(String?x)?{
????????????System.out.println(x);
????????}
????}??
????public?static?void?PrintString(String?s,?
????????????Print?print)?{
????????print.print(s);
????}?
????public?static?void?main(String\[\]?args)?{
????????PrintString("test",?new?Lambda().new?Lambda$$0());
????}
}

異或是這種匿名內部類實現，代碼如下所示:

@FunctionalInterface
interface?Print<T>?{
????public?void?print(T?x);
}
public?class?Lambda?{???
????public?static?void?PrintString(String?s,?
????????????Print?print)?{
????????print.print(s);
????}
????public?static?void?main(String\[\]?args)?{
????????PrintString("test",?new?Print()?{
????????????@Override
????????????public?void?print(String?x)?{
????????????????System.out.println(x);
????????????}
????????});
????}
}

上面的代碼，除了在代碼長度上長了點外，與用Lambda表達式實現的代碼運行結果是一樣的，那么Java?8到底是用什么方式實現的呢??是不是上面三種實現方式中的一種呢，你也許覺的自已想的是對的，其實本來也就是對的，在Java?8中采用的是內部類來實現Lambda表達式

那么Lambda表達式到底是如何實現的呢？

為了探究Lambda表達式是如何實現的，就得需要研究Lambda表過式最終轉化成的字節(jié)碼文件，這就需要jdk的bin目錄下的一個字節(jié)碼查看工具及反編譯工具

javap?-p?Lambda.class

上面命令中的-p表示輸出所有類及成員，運行上面的命令后，得的結果如下所示:

Compiled?from?"Lambda.java"
public?class?Lambda?{
??public?Lambda();
??public?static?void?PrintString(java.lang.String,?Print);
??public?static?void?main(java.lang.String\[\]);
??private?static?void?lambda$0(java.lang.String);
}

由上面的代碼可以看出編譯器會根據Lambda表達式生成一個私有的靜態(tài)函數，注意，在這里說的是生成，而不是等價。

private?static?void?lambda$0(java.lang.String);

為了驗證上面的轉化是否正確??我們在代碼中定義一個lambda$0這個的函數，最終代碼如下所示：

@FunctionalInterface
interface?Print<T>?{
????public?void?print(T?x);
}

public?class?Lambda?{???
????public?static?void?PrintString(String?s,?
????????????Print?print)?{
????????print.print(s);
????}
????private?static?void?lambda$0(String?s)?{
????}
????public?static?void?main(String\[\]?args)?{
????????PrintString("test",?(x)?->?System.out.println(x));
????}
}

上面的代碼在編譯時不會報錯，但是運行時就會報錯，因為存在兩個lambda$0函數，如下所示，是運行時的錯誤：

Exception?in?thread?"main"?java.lang.ClassFormatError:?Duplicate?method?name&signature?in?class?file?Lambda
????at?java.lang.ClassLoader.defineClass1(Native?Method)
????at?java.lang.ClassLoader.defineClass(ClassLoader.java:760)
????at?java.security.SecureClassLoader.defineClass(SecureClassLoader.java:142)
????at?java.net.URLClassLoader.defineClass(URLClassLoader.java:467)
????at?java.net.URLClassLoader.access$100(URLClassLoader.java:73)
????at?java.net.URLClassLoader$1.run(URLClassLoader.java:368)
????at?java.net.URLClassLoader$1.run(URLClassLoader.java:362)
????at?java.security.AccessController.doPrivileged(Native?Method)
????at?java.net.URLClassLoader.findClass(URLClassLoader.java:361)
????at?java.lang.ClassLoader.loadClass(ClassLoader.java:424)
????at?sun.misc.Launcher$AppClassLoader.loadClass(Launcher.java:331)
????at?java.lang.ClassLoader.loadClass(ClassLoader.java:357)
????at?sun.launcher.LauncherHelper.checkAndLoadMain(LauncherHelper.java:495)

通過javap對上述錯誤代碼進行反編譯，反編譯之后輸出的類的成員如下所示

Compiled?from?"Lambda.java"
public?class?Lambda?{
??public?Lambda();
??public?static?void?PrintString(java.lang.String,?Print);
??private?static?void?lambda$0(java.lang.String);
??public?static?void?main(java.lang.String\[\]);
??private?static?void?lambda$0(java.lang.String);
}

會發(fā)現lambda$0出現了兩次，那么在代碼運行的時候，就不知道去調用哪個，因此就會拋錯。

有了上面的內容，可以知道的是Lambda表達式在Java?8中首先會生成一個私有的靜態(tài)函數，這個私有的靜態(tài)函數干的就是Lambda表達式里面的內容,因此上面的代碼初步可以轉化成如下所示的代碼

@FunctionalInterface
interface?Print<T>?{
????public?void?print(T?x);
}
public?class?Lambda?{???
????public?static?void?PrintString(String?s,?Print?print)?{
????????print.print(s);
????}
????
????private?static?void?lambda$0(String?x)?{
????????System.out.println(x);
????}
????
????public?static?void?main(String\[\]?args)?{
????????PrintString("test",?/\*\*lambda?expression\*\*/);
????}
}

轉化成上面的形式之后，那么如何實現調用靜態(tài)的lambda$0函數呢，在這里可以在以下方法打上斷點，可以發(fā)現在有l(wèi)ambda表達式的地方，運行時會進入這個函數

public?static?CallSite?metafactory(MethodHandles.Lookup?caller,
???????????????????????????????????????String?invokedName,
???????????????????????????????????????MethodType?invokedType,
???????????????????????????????????????MethodType?samMethodType,
???????????????????????????????????????MethodHandle?implMethod,
???????????????????????????????????????MethodType?instantiatedMethodType)
????????????throws?LambdaConversionException?{
????????AbstractValidatingLambdaMetafactory?mf;
????????mf?\=?new?InnerClassLambdaMetafactory(caller,?invokedType,
?????????????????????????????????????????????invokedName,?samMethodType,
?????????????????????????????????????????????implMethod,?instantiatedMethodType,
?????????????????????????????????????????????false,?EMPTY\_CLASS\_ARRAY,?EMPTY\_MT\_ARRAY);
????????mf.validateMetafactoryArgs();
????????return?mf.buildCallSite();
}

在這個函數中可以發(fā)現為Lambda表達式生成了一個內部類，為了驗證是否生成內部類，可以在運行時加上-Djdk.internal.lambda.dumpProxyClasses，加上這個參數后，運行時，會將生成的內部類class碼輸出到一個文件中

final?class?Lambda$$Lambda$1?implements?Print?{
??private?Lambda$$Lambda$1();
??public?void?print(java.lang.Object);
}

如果運行javap?-c?-p?則結果如下

final?class?Lambda$$Lambda$1?implements?Print?{
??private?Lambda$$Lambda$1();
????Code:
???????0:?aload\_0
???????1:?invokespecial?#10?????????????????//?Method?java/lang/Object."":()V
???????4:?return

??public?void?print(java.lang.Object);
????Code:
???????0:?aload\_1
???????1:?checkcast?????#14?????????????????//?class?java/lang/String
???????4:?invokestatic??#20?????????????????//?Method?Lambda.lambda$0:(Ljava/lang/String;)V
???????7:?return
}

通過上面的字節(jié)碼指令可以發(fā)現實現上調用的是Lambda.lambda$0這個私有的靜態(tài)方法

因此最終的Lambda表達式等價于以下形式：

@FunctionalInterface
interface?Print<T>?{
????public?void?print(T?x);
}
public?class?Lambda?{???
????public?static?void?PrintString(String?s,?Print?print)?{
????????print.print(s);
????}
????private?static?void?lambda$0(String?x)?{
????????System.out.println(x);
????}
????final?class?$Lambda$1?implements?Print{
????????@Override
????????public?void?print(Object?x)?{
????????????lambda$0((String)x);
????????}
????}
????public?static?void?main(String\[\]?args)?{
????????PrintString("test",?new?Lambda().new?$Lambda$1());
????}
}

至此，關于 Lambda 表達式實現原理的內容已經講完了，希望能夠幫助到大家！