java - 什么是 Java 本地类中的捕获变量

Question

Local Classes 的 Java 文档说:

In addition, a local class has access to local variables. However, a local class can only access local variables that are declared final. When a local class accesses a local variable or parameter of the enclosing block, it captures that variable or parameter. For example, the PhoneNumber constructor can access the local variable numberLength because it is declared final; numberLength is a captured variable.

什么是捕获变量，它的用途是什么，为什么需要它？请帮助我理解它的概念。

Answer 1

What is captured variable,what is its use and why is that needed?

捕获的变量是已复制的变量，因此可以在嵌套类中使用。必须复制它的原因是对象可能超出当前上下文。它必须是 final(或者在 Java 8 中实际上是 final)，所以不会混淆是否会看到变量的更改(因为它们不会)

注意:Groovy 确实有此规则，对局部变量的更改可能意味着对封闭类中值的更改，如果涉及多个线程，这尤其令人困惑。

捕获变量的例子。

public void writeToDataBase(final Object toWrite) {
    executor.submit(new Runnable() {
        public void run() {
             writeToDBNow(toWrite);
        }
    });
    // if toWrite were mutable and you changed it now, what would happen !?
}
// after the method returns toWrite no longer exists for the this thread...