java lambda 原理分析
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Java lambda 一眼看上去有点像匿名内部类的简化形式，但是二者确有着本质的差别。
匿名内部类经编译后会生成对应的class文件，格式为
XXX$n.class
；
而lambda代码经过编译后生成一个private方法，方法名格式为
lambda$main$n
。
// Application.main 方法中代码
ArrayList<String> list = CollectionUtil.newArrayList("I", "love", "you");
list.forEach(new Consumer<String>() {
@Override
public void accept(String s) {
System.out.println(s);
}
});
list.forEach(System.out::println);
以上代码就会产生一个Application$1.class文件和一个lambda$main$0的方法。既然lambda实现不是内部类，那么在lambda中this就代表的当前所在类实例。
// Application.main 方法中代码
ArrayList<String> list = CollectionUtil.newArrayList("I", "love", "you");
list.forEach(item -> {
System.out.println(item);
});
通过javap -c -p Application.class查看以上代码对应的字节码：
Constant pool:
#1 = Methodref #12.#36 // java/lang/Object."<init>":()V
#2 = Class #37 // java/lang/String
#3 = String #38 // I
#4 = String #39 // love
#5 = String #40 // you
#6 = Methodref #41.#42 // cn/hutool/core/collection/CollectionUtil.newArrayList:([Ljava/lang/Object;)Ljava/util/ArrayList;
#7 = InvokeDynamic #0:#48 // #0:accept:()Ljava/util/function/Consumer;
#8 = Methodref #49.#50 // java/util/ArrayList.forEach:(Ljava/util/function/Consumer;)V
#9 = Fieldref #51.#52 // java/lang/System.out:Ljava/io/PrintStream;
#10 = Methodref #53.#54 // java/io/PrintStream.println:(Ljava/lang/String;)V
#11 = Class #55 // com/luo/demo/Application
#12 = Class #56 // java/lang/Object
#13 = Utf8 <init>
#14 = Utf8 ()V
#15 = Utf8 Code
#16 = Utf8 LineNumberTable
#17 = Utf8 LocalVariableTable
#18 = Utf8 this
#19 = Utf8 Lcom/luo/demo/Application;
#20 = Utf8 main
#21 = Utf8 ([Ljava/lang/String;)V
#22 = Utf8 args
#23 = Utf8 [Ljava/lang/String;
#24 = Utf8 list
#25 = Utf8 Ljava/util/ArrayList;
#26 = Utf8 LocalVariableTypeTable
#27 = Utf8 Ljava/util/ArrayList<Ljava/lang/String;>;
#28 = Utf8 lambda$main$0
#29 = Utf8 (Ljava/lang/String;)V
#30 = Utf8 item
#31 = Utf8 Ljava/lang/String;
#32 = Utf8 SourceFile
#33 = Utf8 Application.java
#34 = Utf8 RuntimeVisibleAnnotations
#35 = Utf8 Lorg/springframework/boot/autoconfigure/SpringBootApplication;
#36 = NameAndType #13:#14 // "<init>":()V
#37 = Utf8 java/lang/String
#38 = Utf8 I
#39 = Utf8 love
#40 = Utf8 you
#41 = Class #57 // cn/hutool/core/collection/CollectionUtil
#42 = NameAndType #58:#59 // newArrayList:([Ljava/lang/Object;)Ljava/util/ArrayList;
#43 = Utf8 BootstrapMethods
#44 = MethodHandle #6:#60 // invokestatic java/lang/invoke/LambdaMetafactory.metafactory:(Ljava/lang/invoke/MethodHandles$Lookup;Ljava/lang/String;Ljava/lang/invoke/MethodType;Ljava/lang/invoke/MethodType;Ljava/lang/invoke/MethodHandle;Ljava/lang/invoke/MethodType;)Ljava/lang/invoke/CallSite;
#45 = MethodType #61 // (Ljava/lang/Object;)V
#46 = MethodHandle #6:#62 // invokestatic com/luo/demo/Application.lambda$main$0:(Ljava/lang/String;)V
#47 = MethodType #29 // (Ljava/lang/String;)V
#48 = NameAndType #63:#64 // accept:()Ljava/util/function/Consumer;
#49 = Class #65 // java/util/ArrayList
#50 = NameAndType #66:#67 // forEach:(Ljava/util/function/Consumer;)V
#51 = Class #68 // java/lang/System
#52 = NameAndType #69:#70 // out:Ljava/io/PrintStream;
#53 = Class #71 // java/io/PrintStream
#54 = NameAndType #72:#29 // println:(Ljava/lang/String;)V
#55 = Utf8 com/luo/demo/Application
#56 = Utf8 java/lang/Object
#57 = Utf8 cn/hutool/core/collection/CollectionUtil
#58 = Utf8 newArrayList
#59 = Utf8 ([Ljava/lang/Object;)Ljava/util/ArrayList;
#60 = Methodref #73.#74 // java/lang/invoke/LambdaMetafactory.metafactory:(Ljava/lang/invoke/MethodHandles$Lookup;Ljava/lang/String;Ljava/lang/invoke/MethodType;Ljava/lang/invoke/MethodType;Ljava/lang/invoke/MethodHandle;Ljava/lang/invoke/MethodType;)Ljava/lang/invoke/CallSite;
#61 = Utf8 (Ljava/lang/Object;)V
#62 = Methodref #11.#75 // com/luo/demo/Application.lambda$main$0:(Ljava/lang/String;)V
#63 = Utf8 accept
#64 = Utf8 ()Ljava/util/function/Consumer;
#65 = Utf8 java/util/ArrayList
#66 = Utf8 forEach
#67 = Utf8 (Ljava/util/function/Consumer;)V
#68 = Utf8 java/lang/System
#69 = Utf8 out
#70 = Utf8 Ljava/io/PrintStream;
#71 = Utf8 java/io/PrintStream
#72 = Utf8 println
#73 = Class #76 // java/lang/invoke/LambdaMetafactory
#74 = NameAndType #77:#81 // metafactory:(Ljava/lang/invoke/MethodHandles$Lookup;Ljava/lang/String;Ljava/lang/invoke/MethodType;Ljava/lang/invoke/MethodType;Ljava/lang/invoke/MethodHandle;Ljava/lang/invoke/MethodType;)Ljava/lang/invoke/CallSite;
#75 = NameAndType #28:#29 // lambda$main$0:(Ljava/lang/String;)V
#76 = Utf8 java/lang/invoke/LambdaMetafactory
#77 = Utf8 metafactory
#78 = Class #83 // java/lang/invoke/MethodHandles$Lookup
#79 = Utf8 Lookup
#80 = Utf8 InnerClasses
#81 = Utf8 (Ljava/lang/invoke/MethodHandles$Lookup;Ljava/lang/String;Ljava/lang/invoke/MethodType;Ljava/lang/invoke/MethodType;Ljava/lang/invoke/MethodHandle;Ljava/lang/invoke/MethodType;)Ljava/lang/invoke/CallSite;
#82 = Class #84 // java/lang/invoke/MethodHandles
#83 = Utf8 java/lang/invoke/MethodHandles$Lookup
#84 = Utf8 java/lang/invoke/MethodHandles
{
public com.luo.demo.Application();
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 12: 0
LocalVariableTable:
Start Length Slot Name Signature
0 5 0 this Lcom/luo/demo/Application;
public static void main(java.lang.String[]);
descriptor: ([Ljava/lang/String;)V
flags: ACC_PUBLIC, ACC_STATIC
Code:
stack=4, locals=2, args_size=1
0: iconst_3
1: anewarray #2 // class java/lang/String
4: dup
5: iconst_0
6: ldc #3 // String I
8: aastore
9: dup
10: iconst_1
11: ldc #4 // String love
13: aastore
14: dup
15: iconst_2
16: ldc #5 // String you
18: aastore
19: invokestatic #6 // Method cn/hutool/core/collection/CollectionUtil.newArrayList:([Ljava/lang/Object;)Ljava/util/ArrayList;
22: astore_1
23: aload_1
24: invokedynamic #7, 0 // InvokeDynamic #0:accept:()Ljava/util/function/Consumer;
29: invokevirtual #8 // Method java/util/ArrayList.forEach:(Ljava/util/function/Consumer;)V
32: return
LineNumberTable:
line 15: 0
line 16: 23
line 19: 32
LocalVariableTable:
Start Length Slot Name Signature
0 33 0 args [Ljava/lang/String;
23 10 1 list Ljava/util/ArrayList;
LocalVariableTypeTable:
Start Length Slot Name Signature
23 10 1 list Ljava/util/ArrayList<Ljava/lang/String;>;
private static void lambda$main$0(java.lang.String);
descriptor: (Ljava/lang/String;)V
flags: ACC_PRIVATE, ACC_STATIC, ACC_SYNTHETIC
Code:
stack=2, locals=1, args_size=1
0: getstatic #9 // Field java/lang/System.out:Ljava/io/PrintStream;
3: aload_0
4: invokevirtual #10 // Method java/io/PrintStream.println:(Ljava/lang/String;)V
7: return
LineNumberTable:
line 17: 0
line 18: 7
LocalVariableTable:
Start Length Slot Name Signature
0 8 0 item Ljava/lang/String;
}
通过字节码可以看出，调用lambda方法时使用了 invokedynamic
，该字节码命令是为了支持动态语言特性而在Java7中新增的。Java的lambda表达式实现上也就借助于 invokedynamic
命令。
字节码中每一处含有invokeDynamic指令的位置都称为 动态调用点 ，这条指令的第一个参数不再是代表方法调用符号引用的CONSTANT_Methodref_info常亮，而是变成为JDK7新加入的CONSTANT_InvokeDynamic_info常量，从这个新常量中可得到3项信息：引导方法（Bootstrap Method，此方法存放在新增的BootstrapMethods属性中）、方法类型和名称。引导方法是有固定的参数，并且返回值是java.lang.invoke.CallSite对象，这个代表真正要执行的目标方法调用。根据CONSTANT_InvokeDynamic_info常量中提供的信息，虚拟机可以找到并执行引导方法，从而获得一个CallSite对象，最终调用要执行的目标方法。
从上述mian方法的字节码可见，有一个invokeDynamic指令，他的参数为第7项常量（第二个值为0的参数HotSpot中用不到，占位符）。 invokedynamic #7, 0 // InvokeDynamic #0:accept:()Ljava/util/function/Consumer;
常量池中第7项是 #7 = InvokeDynamic #0:#48 // #0:accept:()Ljava/util/function/Consumer;
，说明它是一项CONSTANT_InvokeDynamic_info常量，常量值中前面的#0表示引导方法取BootstrapMethods属性表的第0项，而后面的#48表示引用第48项类型为CONSTANAT_NameAndType_info的常量，从这个常量中可以获取方法名称和描述符，即accept方法。
BootstrapMethods:
0: #44 invokestatic java/lang/invoke/LambdaMetafactory.metafactory:(Ljava/lang/invoke/MethodHandles$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:
#45 (Ljava/lang/Object;)V
#46 invokestatic com/luo/demo/Application.lambda$main$0:(Ljava/lang/String;)V
#47 (Ljava/lang/String;)V
上图是在lambda代码中打断点时的调用栈信息，如果在这里的lambda中打印当前所属class，就是Application类，也印证了前面分析的lambda代码会生成一个private方法。
从调用栈的信息来看，是在accept方法中调用lambda对应的private方法（ambda$main$0）的，但是这里的accept方法是属于什么对象呢？从图中看是一串数字字符串，这里可以理解成一个Consumer接口的实现类即可，每个lambda表达式可以理解成在一个新的Consumer实现类中调用的即可。使用命令jmap -histo查看JVM进程类和对象信息可以看到这一行信息： 600: 1 16 com.luo.demo.Application$$Lambda$5/1615039080
参考资料：
1、https://blog.csdn.net/zxhoo/article/details/38387141
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