Thread中join方法源码阅读

以JDK8为例，共3个join方法

一、核心join方法

public final synchronized void join(long millis)
    throws InterruptedException {
        long base = System.currentTimeMillis();
        long now = 0;
　　　　　// 不合法参数
        if (millis < 0) {
            throw new IllegalArgumentException("timeout value is negative");
        }
　　　　　// 等待时间为0
        if (millis == 0) {
            while (isAlive()) {
                wait(0);
            }
        } else {
　　　　　// 利用System.currentTimeMillis()计算剩余等待时间
            while (isAlive()) {
                long delay = millis - now;
                if (delay <= 0) {
                    break;
                }
                wait(delay);
                now = System.currentTimeMillis() - base;
            }
        }
    }

二、更精细的join方法

public final synchronized void join(long millis, int nanos)
    throws InterruptedException {

        if (millis < 0) {
            throw new IllegalArgumentException("timeout value is negative");
        }

        if (nanos < 0 || nanos > 999999) {
            throw new IllegalArgumentException(
                                "nanosecond timeout value out of range");
        }

        if (nanos >= 500000 || (nanos != 0 && millis == 0)) {
            millis++;
        }

        join(millis);
    }

三、最常用的join方法

public final void join() throws InterruptedException {
        join(0);
    }