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<String> supplier = () -> {
return para + para2;
};
System.out.println(supplier.get());
}
}
对应的jvm 机器码是:
private static java.lang.String lambda$test$0(java.lang.String, java.lang.String);
descriptor: (Ljava/lang/String;Ljava/lang/String;)Ljava/lang/String;
flags: ACC_PRIVATE, ACC_STATIC, ACC_SYNTHETIC
Code:
stack=2, locals=2, args_size=2
0: new #12 // class java/lang/StringBuilder
3: dup
4: invokespecial #13 // Method java/lang/StringBuilder."<init>":()V
7: aload_0
8: invokevirtual #14 // Method java/lang/StringBuilder.append:(Ljava/lang/String;)Ljava/lang/StringBuilder;
11: aload_1
12: invokevirtual #14 // Method java/lang/StringBuilder.append:(Ljava/lang/String;)Ljava/lang/StringBuilder;
15: invokevirtual #15 // Method java/lang/StringBuilder.toString:()Ljava/lang/String;
18: areturn
LineNumberTable:
line 16: 0
通过方法签名可以知道,如果一个类没有带 this,被编译成了一个静态内部类方法。
带 this:
import java.util.function.Supplier;
public class LambdaTest {
public static void main(String[] args) {
new LambdaTest().test();
}
public void test() {
String para = "abc";
String para2 = "abc";
System.out.println(this);
Supplier<String> supplier = () -> {
System.out.println(this);
return para + para2;
};
System.out.println(supplier.get());
}
}
对应的 jvm 机器码:
private java.lang.String lambda$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。