JDK8中的effectively final

However, starting in Java SE 8, a local class can access local variables and parameters of the enclosing block that are final or effectively final. A variable or parameter whose value is never changed after it is initialized is effectively final.
https://docs.oracle.com/javase/tutorial/java/javaOO/localclasses.html

简单来说，如果只读一个外部变量，不再需要指定成final了。
而在之前，常见的匿名内部类访问外部局部变量必须指定成final。

下面这段代码1.8就可以执行，1.7就会编译报错。

public void test(){
    int aa = 10;
    final int bb = 20;
    Thread t1 = new Thread(new Runnable() {
        @Override
        public void run() {
            System.out.println(aa+bb); 
        }
    });
}

1.7报错内容：Error:(22, 36) java: 从内部类中访问本地变量aa; 需要被声明为最终类型

当你要修改的外部变量时，在1.8下也会直接报错：

public void test(){
    int aa = 10;
    final int bb = 20;
    Thread t1 = new Thread(new Runnable() {
        @Override
        public void run() {
            System.out.println(aa+bb);
        }
    });

    Thread t2 = new Thread(new Runnable() {
        @Override
        public void run() {
            aa = 20; //Error
        }
    });
}

1.8报错内容Error:(34, 17) java: 从内部类引用的本地变量必须是最终变量或实际上的最终变量
(needs to be final or effectively final)