这几天复习了 lambda ，发现有个细节，十分难以理解，那就是 lambda 里的 this指针。

Lambda 里的this指针指向其所属的内部类， 是怎么实现的呢？

写了一个例子，作为测试:

import java.util.function.Supplier;

public class LambdaTest {

public static void main(String[] args) {

new LambdaTest().test();

}

public void test() {

String para = "abc";

String para2 = "abc";

System.out.println(this);

Supplier supplier = () -> {

return para + para2;

};

System.out.println(supplier.get());

}

}

对应的jvm 机器码是:

private static java.lang.String lambda$test$0(java.lang.String, java.lang.String);

descriptor: (Ljava/lang/String;Ljava/lang/String;)Ljava/lang/String;

flags: ACC_PRIVATE, ACC_STATIC, ACC_SYNTHETIC

Code:

stack=2, locals=2, args_size=2

0: new #12 // class java/lang/StringBuilder

3: dup

4: invokespecial #13 // Method java/lang/StringBuilder."":()V

7: aload_0

8: invokevirtual #14 // Method java/lang/StringBuilder.append:(Ljava/lang/String;)Ljava/lang/StringBuilder;

11: aload_1

12: invokevirtual #14 // Method java/lang/StringBuilder.append:(Ljava/lang/String;)Ljava/lang/StringBuilder;

15: invokevirtual #15 // Method java/lang/StringBuilder.toString:()Ljava/lang/String;

18: areturn

LineNumberTable:

line 16: 0

通过方法签名可以知道，如果一个类没有带 this,被编译成了一个静态内部类方法。

带 this：

import java.util.function.Supplier;

public class LambdaTest {

public static void main(String[] args) {

new LambdaTest().test();

}

public void test() {

String para = "abc";

String para2 = "abc";

System.out.println(this);

Supplier supplier = () -> {

System.out.println(this);

return para + para2;

};

System.out.println(supplier.get());

}

}

对应的 jvm 机器码:

private java.lang.String lambda$test$0(java.lang.String, java.lang.String);

descriptor: (Ljava/lang/String;Ljava/lang/String;)Ljava/lang/String;

flags: ACC_PRIVATE, ACC_SYNTHETIC

Code:

stack=2, locals=3, args_size=3

0: getstatic #6 // Field java/lang/System.out:Ljava/io/PrintStream;

3: aload_0

4: invokevirtual #7 // Method java/io/PrintStream.println:(Ljava/lang/Object;)V

7: new #12 // class java/lang/StringBuilder

10: dup

11: invokespecial #13 // Method java/lang/StringBuilder."":()V

14: aload_1

15: invokevirtual #14 // Method java/lang/StringBuilder.append:(Ljava/lang/String;)Ljava/lang/StringBuilder;

18: aload_2

19: invokevirtual #14 // Method java/lang/StringBuilder.append:(Ljava/lang/String;)Ljava/lang/StringBuilder;

22: invokevirtual #15 // Method java/lang/StringBuilder.toString:()Ljava/lang/String;

25: areturn

LineNumberTable:

line 15: 0

line 16: 7

lambda 被编译成了一种内部类！这就能说通了。

结论:

lambda一般情况下会被编译成静态匿名方法，引用的外部变量以参数的方式传递。

如果 lambda 里使用了this 指标，则被编译为匿名内部方法，以让 this 指针指向lambda 外部类。