您需要参考SE8 15.27:
15.27.3。一个Lambda表达式
lambda表达式是在分配上下文兼容,调用 上下文,或者与目标类型T铸造上下文如果T是一个功能性 接口类型(§9.8)和类型的表达是一致与 从T派生的地面目标类型的函数类型。
运行时处理使用棘手的事情与invokedynamic
。让检查一些代码:
import java.util.function.*;
class R implements Runnable {
public void run() { System.out.println("there"); }
}
public class L {
public static void execute(Runnable r) {
System.out.println(r.getClass());
r.run();
}
public static void main(String []a) {
execute(new R()); // subclass
execute(new Runnable() { // anonymous subclass
public void run() { System.out.println("elsewhere"); }
});
execute(() -> System.out.println("here")); // lambda
}
}
执行给出:
> java L
class R
there
class L$1
elsewhere
class L$$Lambda$1/791452441
here
对于前两个没有惊讶的是,类是通过方法execute
接收到的对象的(在给定的顺序)R
(该(Runnable
的子类型),L$1
(Runnable
和L$$Lambda$1/791452441
的匿名子类型(Runnable
的子类型在运行时从lambda构建)。请注意,在lambda的情况下没有.class
文件,该类型在运行时由特殊结构构造。让我们来看看字节码:
> javap -c -v L
Classfile /private/tmp/L.class
Last modified 1 août 2017; size 1234 bytes
MD5 checksum 9680a2bc143d25344979bae00fff3db7
Compiled from "L.java"
public class L
minor version: 0
major version: 52
flags: ACC_PUBLIC, ACC_SUPER
Constant pool:
#1 = Methodref #15.#28 // java/lang/Object."<init>":()V
#2 = Fieldref #29.#30 // java/lang/System.out:Ljava/io/PrintStream;
#3 = Methodref #15.#31 // java/lang/Object.getClass:()Ljava/lang/Class;
#4 = Methodref #32.#33 // java/io/PrintStream.println:(Ljava/lang/Object;)V
#5 = InterfaceMethodref #34.#35 // java/lang/Runnable.run:()V
#6 = Class #36 // R
#7 = Methodref #6.#28 // R."<init>":()V
#8 = Methodref #14.#37 // L.execute:(Ljava/lang/Runnable;)V
#9 = Class #38 // L$1
#10 = Methodref #9.#28 // L$1."<init>":()V
#11 = InvokeDynamic #0:#43 // #0:run:()Ljava/lang/Runnable;
#12 = String #44 // here
#13 = Methodref #32.#45 // java/io/PrintStream.println:(Ljava/lang/String;)V
#14 = Class #46 // L
#15 = Class #47 // java/lang/Object
#16 = Utf8 InnerClasses
#17 = Utf8 <init>
#18 = Utf8 ()V
#19 = Utf8 Code
#20 = Utf8 LineNumberTable
#21 = Utf8 execute
#22 = Utf8 (Ljava/lang/Runnable;)V
#23 = Utf8 main
#24 = Utf8 ([Ljava/lang/String;)V
#25 = Utf8 lambda$main$0
#26 = Utf8 SourceFile
#27 = Utf8 L.java
#28 = NameAndType #17:#18 // "<init>":()V
#29 = Class #48 // java/lang/System
#30 = NameAndType #49:#50 // out:Ljava/io/PrintStream;
#31 = NameAndType #51:#52 // getClass:()Ljava/lang/Class;
#32 = Class #53 // java/io/PrintStream
#33 = NameAndType #54:#55 // println:(Ljava/lang/Object;)V
#34 = Class #56 // java/lang/Runnable
#35 = NameAndType #57:#18 // run:()V
#36 = Utf8 R
#37 = NameAndType #21:#22 // execute:(Ljava/lang/Runnable;)V
#38 = Utf8 L$1
#39 = Utf8 BootstrapMethods
#40 = MethodHandle #6:#58 // invokestatic java/lang/invoke/LambdaMetafactory.metafactory:(Ljava/lang/invoke/MethodHandles$Lookup;Ljava/lang/String;Ljava/lang/invoke/MethodType;Ljava/lang/invoke/MethodType;Ljava/lang/invoke/MethodHandle;Ljava/lang/invoke/MethodType;)Ljava/lang/invoke/CallSite;
#41 = MethodType #18 // ()V
#42 = MethodHandle #6:#59 // invokestatic L.lambda$main$0:()V
#43 = NameAndType #57:#60 // run:()Ljava/lang/Runnable;
#44 = Utf8 here
#45 = NameAndType #54:#61 // println:(Ljava/lang/String;)V
#46 = Utf8 L
#47 = Utf8 java/lang/Object
#48 = Utf8 java/lang/System
#49 = Utf8 out
#50 = Utf8 Ljava/io/PrintStream;
#51 = Utf8 getClass
#52 = Utf8 ()Ljava/lang/Class;
#53 = Utf8 java/io/PrintStream
#54 = Utf8 println
#55 = Utf8 (Ljava/lang/Object;)V
#56 = Utf8 java/lang/Runnable
#57 = Utf8 run
#58 = Methodref #62.#63 // java/lang/invoke/LambdaMetafactory.metafactory:(Ljava/lang/invoke/MethodHandles$Lookup;Ljava/lang/String;Ljava/lang/invoke/MethodType;Ljava/lang/invoke/MethodType;Ljava/lang/invoke/MethodHandle;Ljava/lang/invoke/MethodType;)Ljava/lang/invoke/CallSite;
#59 = Methodref #14.#64 // L.lambda$main$0:()V
#60 = Utf8 ()Ljava/lang/Runnable;
#61 = Utf8 (Ljava/lang/String;)V
#62 = Class #65 // java/lang/invoke/LambdaMetafactory
#63 = NameAndType #66:#69 // metafactory:(Ljava/lang/invoke/MethodHandles$Lookup;Ljava/lang/String;Ljava/lang/invoke/MethodType;Ljava/lang/invoke/MethodType;Ljava/lang/invoke/MethodHandle;Ljava/lang/invoke/MethodType;)Ljava/lang/invoke/CallSite;
#64 = NameAndType #25:#18 // lambda$main$0:()V
#65 = Utf8 java/lang/invoke/LambdaMetafactory
#66 = Utf8 metafactory
#67 = Class #71 // java/lang/invoke/MethodHandles$Lookup
#68 = Utf8 Lookup
#69 = Utf8 (Ljava/lang/invoke/MethodHandles$Lookup;Ljava/lang/String;Ljava/lang/invoke/MethodType;Ljava/lang/invoke/MethodType;Ljava/lang/invoke/MethodHandle;Ljava/lang/invoke/MethodType;)Ljava/lang/invoke/CallSite;
#70 = Class #72 // java/lang/invoke/MethodHandles
#71 = Utf8 java/lang/invoke/MethodHandles$Lookup
#72 = Utf8 java/lang/invoke/MethodHandles
{
public L();
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 9: 0
public static void execute(java.lang.Runnable);
descriptor: (Ljava/lang/Runnable;)V
flags: ACC_PUBLIC, ACC_STATIC
Code:
stack=2, locals=1, args_size=1
0: getstatic #2 // Field java/lang/System.out:Ljava/io/PrintStream;
3: aload_0
4: invokevirtual #3 // Method java/lang/Object.getClass:()Ljava/lang/Class;
7: invokevirtual #4 // Method java/io/PrintStream.println:(Ljava/lang/Object;)V
10: aload_0
11: invokeinterface #5, 1 // InterfaceMethod java/lang/Runnable.run:()V
16: return
LineNumberTable:
line 11: 0
line 12: 10
line 13: 16
public static void main(java.lang.String[]);
descriptor: ([Ljava/lang/String;)V
flags: ACC_PUBLIC, ACC_STATIC
Code:
stack=2, locals=1, args_size=1
0: new #6 // class R
3: dup
4: invokespecial #7 // Method R."<init>":()V
7: invokestatic #8 // Method execute:(Ljava/lang/Runnable;)V
10: new #9 // class L$1
13: dup
14: invokespecial #10 // Method L$1."<init>":()V
17: invokestatic #8 // Method execute:(Ljava/lang/Runnable;)V
20: invokedynamiC#11, 0 // InvokeDynamiC#0:run:()Ljava/lang/Runnable;
25: invokestatic #8 // Method execute:(Ljava/lang/Runnable;)V
28: return
LineNumberTable:
line 15: 0
line 16: 10
line 19: 20
line 20: 28
}
SourceFile: "L.java"
InnerClasses:
statiC#9; //class L$1
public static final #68= #67 of #70; //Lookup=class java/lang/invoke/MethodHandles$Lookup of class java/lang/invoke/MethodHandles
BootstrapMethods:
0: #40 invokestatic java/lang/invoke/LambdaMetafactory.metafactory:(Ljava/lang/invoke/MethodHandles$Lookup;Ljava/lang/String;Ljava/lang/invoke/MethodType;Ljava/lang/invoke/MethodType;Ljava/lang/invoke/MethodHandle;Ljava/lang/invoke/MethodType;)Ljava/lang/invoke/CallSite;
Method arguments:
#41()V
#42 invokestatic L.lambda$main$0:()V
#41()V
第一个有趣的部分是main
代码:
public static void main(java.lang.String[]);
descriptor: ([Ljava/lang/String;)V
flags: ACC_PUBLIC, ACC_STATIC
Code:
stack=2, locals=1, args_size=1
0: new #6 // class R
3: dup
4: invokespecial #7 // Method R."<init>":()V
7: invokestatic #8 // Method execute:(Ljava/lang/Runnable;)V
10: new #9 // class L$1
13: dup
14: invokespecial #10 // Method L$1."<init>":()V
17: invokestatic #8 // Method execute:(Ljava/lang/Runnable;)V
20: invokedynamiC#11, 0 // InvokeDynamiC#0:run:()Ljava/lang/Runnable;
25: invokestatic #8 // Method execute:(Ljava/lang/Runnable;)V
28: return
正如你可以看到有在显式接口的实现或匿名一个没有任何区别。最后只涉及类命名欺骗(L$1
),但两者都以相同的方式使用,通过invokestatic
。
有趣的案例是涉及invokedynamic
,然后invokestatic
第三(拉姆达一)。请注意,invokestatic
调用与前两次调用(方法run
)相同的方法。
粗略地说,invokedynamic
第一次被称为bootstrap方法被调用来构造CallSite
(参见CallSite in Java API),然后它将被进一步用来执行lambda的代码。见引导来电来访:
BootstrapMethods:
0: #40 invokestatic java/lang/invoke/LambdaMetafactory.metafactory:(Ljava/lang/invoke/MethodHandles$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:
#41()V
#42 invokestatic L.lambda$main$0:()V
#41()V
和代码通过调用现场简称:
#41 = MethodType #18 // ()V
#42 = MethodHandle #6:#59 // invokestatic L.lambda$main$0:()V
#43 = NameAndType #57:#60 // run:()Ljava/lang/Runnable;
#44 = Utf8 here
#45 = NameAndType #54:#61 // println:(Ljava/lang/String;)V
#46 = Utf8 L
#47 = Utf8 java/lang/Object
#48 = Utf8 java/lang/System
#49 = Utf8 out
#50 = Utf8 Ljava/io/PrintStream;
#51 = Utf8 getClass
#52 = Utf8 ()Ljava/lang/Class;
#53 = Utf8 java/io/PrintStream
#54 = Utf8 println
#55 = Utf8 (Ljava/lang/Object;)V
#56 = Utf8 java/lang/Runnable
#57 = Utf8 run
*有*类。这是烟雾和镜子。 – user2864740
这是因为JVM将在内存中为lambda表达式/方法引用表达式的桥接方法创建'Consumer'的子类。 –
@ holi-java我在类路径中找不到任何'.class'文件。您可以简要介绍一下。内存中的 – TheCurious