java8里lambda里的 this 为什么会指向 lamdba 所在的外部类

发表于 | 分类于

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:

1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
16
17
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<String> supplier = () -> {
            return para + para2;
        };
        System.out.println(supplier.get());
    }
}

对应的jvm 机器码是:

1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
16
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."<init>":()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：

1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
16
17
18
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<String> supplier = () -> {
            System.out.println(this);
            return para + para2;
        };
        System.out.println(supplier.get());
    }
}

对应的 jvm 机器码:

1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
16
17
18
19
20
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."<init>":()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。