java内部匿名类

前言:
java基础的编写类的时候,会觉得很匿名内部类很神秘.普通的java文件会被编译成class,那内部类和匿名内部类呢?它们被会存在在哪个物理位置?
本文主要针对匿名内部类.

本文中有ISay和ISay2两个接口,用于测试对比.

ISay.java

public interface ISay {
     void sayHello();
 }

ISay2.java

public interface ISay2 {
     void sayHello222();

     void print();
 }

Program.java

public class Program {
    public static void main(String[] args) {
        ISay say = new ISay() {
            public void sayHello() {
                System.out.println("Hello java!");
            }
        };

        ISay say2 = new ISay() {
            public void sayHello() {
                System.out.println("Hello java!");
            }
        };

        ISay say3 = new ISay() {
            public void sayHello() {
                System.out.println("Hello java!");
            }
        };


        ISay2 say2_1 = new ISay2() {
            public void sayHello222() {
                System.out.println("Hello java!");
            }

            public void print(){
                System.out.print("print");
            }
        };
 
        say.sayHello();
        say2.sayHello();
        say2_1.sayHello222();
        say2_1.print();
    }
}

生成的class文件如下图：
ISay.java

public interface ISay {
     void sayHello();
 }

ISay2.java

public interface ISay2 {
     void sayHello222();

     void print();
 }

Program.java

public class Program {
    public static void main(String[] args) {
        ISay say = new ISay() {
            public void sayHello() {
                System.out.println("Hello java!");
            }
        };

        ISay say2 = new ISay() {
            public void sayHello() {
                System.out.println("Hello java!");
            }
        };

        ISay say3 = new ISay() {
            public void sayHello() {
                System.out.println("Hello java!");
            }
        };


        ISay2 say2_1 = new ISay2() {
            public void sayHello222() {
                System.out.println("Hello java!");
            }

            public void print(){
                System.out.print("print");
            }
        };
 
        say.sayHello();
        say2.sayHello();
        say2_1.sayHello222();
        say2_1.print();
    }
}

生成的class文件如下图：

6863841.png

发现会有Program$1.class等文件,其实这就是匿名类的class文件.

使用jclasslib查看Program.class字节码
###是注释，不是字节码的内容

new #2 <NewInterface/Program$1>                      ###对应代码ISay say = new Program$1() ,Program$1是ISay的实现。
dup                                                                                                                                                                                                       
invokespecial #3 <NewInterface/Program$1.<init>>
astore_1                                             ###存放引用在变量1中

new #4 <NewInterface/Program$2>
dup
invokespecial #5 <NewInterface/Program$2.<init>>
astore_2

new #6 <NewInterface/Program$3>
dup
invokespecial #7 <NewInterface/Program$3.<init>>
astore_3

new #8 <NewInterface/Program$4>
dup
invokespecial #9 <NewInterface/Program$4.<init>>
astore 4

aload_1                                                      ###加载本地变量1的引用，即上述的NewInterface/Program$1
invokeinterface #10 <NewInterface/ISay.sayHello> count 1     ###调用方法，对象是本地变量1的实例。

aload_2
invokeinterface #10 <NewInterface/ISay.sayHello> count 1

aload 4                                                      ###加载本地变量4的引用，即上述的NewInterface/Program$4
invokeinterface #11 <NewInterface/ISay2.sayHello222> count 1 ###调用方法，对象是本地变量4的实例。

aload 4
invokeinterface #12 <NewInterface/ISay2.print> count 1
return

内部类跟匿名内部类的class生成规则差不多,只不过匿名内部类是由编译器命令,内部类是直接指定.
例如:Program中有一个内部类叫Book,那么生成的class文件是Program$Book.class