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Java8 Lambda表达式原理扫盲

背景

在使用Lamdba表达式，一直以为是内部类的方式实现的，但是一想如果每次调用都实例化一个内部类，性能肯定不好，难道Java里的lambda表达式真的是这么实现的吗？也许是该研究下原理了。

正文

  1. 测试代码：
public class Test{

	public void test() {

	    Runnable r = () -> System.out.println(123);

	    r.run();

	}

}

执行编译命令javac -g Test.java，得到class文件。

  1. 查看字节码

查看字节码javap -p -verbose Test得到：

Classfile /Users/liushijie/learn/Test.class

  Last modified Nov 20, 2018; size 1058 bytes

  MD5 checksum febbe61fdc1f4564d2e039067752d6fc

  Compiled from "Test.java"

public class Test

  minor version: 0

  major version: 52

  flags: ACC_PUBLIC, ACC_SUPER

Constant pool:

   #1 = Methodref          #7.#21         // java/lang/Object."<init>":()V

   #2 = InvokeDynamic      #0:#26         // #0:run:()Ljava/lang/Runnable;

   #3 = InterfaceMethodref #27.#28        // java/lang/Runnable.run:()V

   #4 = Fieldref           #29.#30        // java/lang/System.out:Ljava/io/PrintStream;

   #5 = Methodref          #31.#32        // java/io/PrintStream.println:(I)V

   #6 = Class              #33            // Test

   #7 = Class              #34            // java/lang/Object

   #8 = Utf8               <init>

   #9 = Utf8               ()V

  #10 = Utf8               Code

  #11 = Utf8               LineNumberTable

  #12 = Utf8               LocalVariableTable

  #13 = Utf8               this

  #14 = Utf8               LTest;

  #15 = Utf8               test

  #16 = Utf8               r

  #17 = Utf8               Ljava/lang/Runnable;

  #18 = Utf8               lambda$test$0

  #19 = Utf8               SourceFile

  #20 = Utf8               Test.java

  #21 = NameAndType        #8:#9          // "<init>":()V

  #22 = Utf8               BootstrapMethods

  #23 = MethodHandle       #6:#35         // invokestatic java/lang/invoke/LambdaMetafactory.metafactory:(Ljava/lang/invoke/MethodHandles$Lookup;Ljava/lang/String;Ljava/lang/invoke/MethodType;Ljava/lang/invoke/MethodType;Ljava/lang/invoke/MethodHandle;Ljava/lang/invoke/MethodType;)Ljava/lang/invoke/CallSite;

  #24 = MethodType         #9             //  ()V

  #25 = MethodHandle       #6:#36         // invokestatic Test.lambda$test$0:()V

  #26 = NameAndType        #37:#38        // run:()Ljava/lang/Runnable;

  #27 = Class              #39            // java/lang/Runnable

  #28 = NameAndType        #37:#9         // run:()V

  #29 = Class              #40            // java/lang/System

  #30 = NameAndType        #41:#42        // out:Ljava/io/PrintStream;

  #31 = Class              #43            // java/io/PrintStream

  #32 = NameAndType        #44:#45        // println:(I)V

  #33 = Utf8               Test

  #34 = Utf8               java/lang/Object

  #35 = Methodref          #46.#47        // java/lang/invoke/LambdaMetafactory.metafactory:(Ljava/lang/invoke/MethodHandles$Lookup;Ljava/lang/String;Ljava/lang/invoke/MethodType;Ljava/lang/invoke/MethodType;Ljava/lang/invoke/MethodHandle;Ljava/lang/invoke/MethodType;)Ljava/lang/invoke/CallSite;

  #36 = Methodref          #6.#48         // Test.lambda$test$0:()V

  #37 = Utf8               run

  #38 = Utf8               ()Ljava/lang/Runnable;

  #39 = Utf8               java/lang/Runnable

  #40 = Utf8               java/lang/System

  #41 = Utf8               out

  #42 = Utf8               Ljava/io/PrintStream;

  #43 = Utf8               java/io/PrintStream

  #44 = Utf8               println

  #45 = Utf8               (I)V

  #46 = Class              #49            // java/lang/invoke/LambdaMetafactory

  #47 = NameAndType        #50:#54        // metafactory:(Ljava/lang/invoke/MethodHandles$Lookup;Ljava/lang/String;Ljava/lang/invoke/MethodType;Ljava/lang/invoke/MethodType;Ljava/lang/invoke/MethodHandle;Ljava/lang/invoke/MethodType;)Ljava/lang/invoke/CallSite;

  #48 = NameAndType        #18:#9         // lambda$test$0:()V

  #49 = Utf8               java/lang/invoke/LambdaMetafactory

  #50 = Utf8               metafactory

  #51 = Class              #56            // java/lang/invoke/MethodHandles$Lookup

  #52 = Utf8               Lookup

  #53 = Utf8               InnerClasses

  #54 = Utf8               (Ljava/lang/invoke/MethodHandles$Lookup;Ljava/lang/String;Ljava/lang/invoke/MethodType;Ljava/lang/invoke/MethodType;Ljava/lang/invoke/MethodHandle;Ljava/lang/invoke/MethodType;)Ljava/lang/invoke/CallSite;

  #55 = Class              #57            // java/lang/invoke/MethodHandles

  #56 = Utf8               java/lang/invoke/MethodHandles$Lookup

  #57 = Utf8               java/lang/invoke/MethodHandles

{

  public Test();

    descriptor: ()V

    flags: ACC_PUBLIC

    Code:

      stack=1, locals=1, args_size=1

         0: aload_0

         1: invokespecial #1                  // Method java/lang/Object."<init>":()V

         4: return

      LineNumberTable:

        line 1: 0

      LocalVariableTable:

        Start  Length  Slot  Name   Signature

            0       5     0  this   LTest;

  public void test();

    descriptor: ()V

    flags: ACC_PUBLIC

    Code:

      stack=1, locals=2, args_size=1

         0: invokedynamic #2,  0              // InvokeDynamic #0:run:()Ljava/lang/Runnable;

         5: astore_1

         6: aload_1

         7: invokeinterface #3,  1            // InterfaceMethod java/lang/Runnable.run:()V

        12: return

      LineNumberTable:

        line 3: 0

        line 4: 6

        line 5: 12

      LocalVariableTable:

        Start  Length  Slot  Name   Signature

            0      13     0  this   LTest;

            6       7     1     r   Ljava/lang/Runnable;

  private static void lambda$test$0();

    descriptor: ()V

    flags: ACC_PRIVATE, ACC_STATIC, ACC_SYNTHETIC

    Code:

      stack=2, locals=0, args_size=0

         0: getstatic     #4                  // Field java/lang/System.out:Ljava/io/PrintStream;

         3: bipush        123

         5: invokevirtual #5                  // Method java/io/PrintStream.println:(I)V

         8: return

      LineNumberTable:

        line 3: 0

}

SourceFile: "Test.java"

InnerClasses:

     public static final #52= #51 of #55; //Lookup=class java/lang/invoke/MethodHandles$Lookup of class java/lang/invoke/MethodHandles

BootstrapMethods:

  0: #23 invokestatic java/lang/invoke/LambdaMetafactory.metafactory:(Ljava/lang/invoke/MethodHandles$Lookup;Ljava/lang/String;Ljava/lang/invoke/MethodType;Ljava/lang/invoke/MethodType;Ljava/lang/invoke/MethodHandle;Ljava/lang/invoke/MethodType;)Ljava/lang/invoke/CallSite;

    Method arguments:

      #24 ()V

      #25 invokestatic Test.lambda$test$0:()V

      #24 ()V

通过字节码文件我们可以看到，编译出来的字节码文件中新增一些玩意：

  1. 常量池里（#2）多了一个以前没有见过的InvokeDynamic指令
  2. 新增了一个静态私有方法：private static void lambda$test$0()，里面的内容正好是lambda表达式里的代码；
  3. 新增了一个BootstrapMethods属性，内部包含一个动态调用点列表，因为测试代码只有一个lambda表达式，所以我们只能看到一个调用点

在运行时有一个链接(link)过程，在JVM层面调用。通过链接操作，调用上面3中的调用点，调用点在动态生成实现了FunctionInterface接口的类，方法中则调用2中新增的lambda$test$0方法，生成类之后通过构造函数实例化一个对象，被调用点持有，调用点有一个字的常量池。在调用invokedynamic指令之前会发生链接过程。下文引自：参考5

里面也有提到过多线程场景，略过不提。

Before the JVM can execute a dynamic call site (an invokedynamic instruction), the call site must first be linked. Linking is accomplished by calling a bootstrap method which is given the static information content of the call site, and which must produce a method handle that gives the behavior of the call site.

总结

通过一些验证和资料检索，大概了解lambda的原理，是使用指令与动态生成的内部类来完成调用，而且正常只会被链接一次。从这个点上来看，对性能是没有什么损失的，可以放心的使用。

问题

自己梳理的比较肤浅，没有深挖最底层的实现。LambdaMetafactory.metafactory动态调用点的链接过程比较长，如果有动态调用的场景应该是可以参考的。翻到过一篇问答（见参考6），暂时没找到合适的场景使用，没有深入下去的动力。

参考

  1. Lambda表达式实现方式
  2. InvokeDynamic指令JSR 292
  3. https://stackoverflow.com/questions/26775676/explicit-use-of-lambdametafactory
  4. https://zhuanlan.zhihu.com/p/27159693
  5. https://docs.oracle.com/javase/8/docs/api/java/lang/invoke/package-summary.html#package.description
  6. https://stackoverflow.com/questions/26775676/explicit-use-of-lambdametafactory
05-11 15:12
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