Join方法
定義:等待被調(diào)用的線程結(jié)束示惊。具體來說,
t.join()
表示阻塞調(diào)用此方法的線程愉镰,直到線程t完成為此米罚,方可繼續(xù)0。樣例一:計(jì)算子線程的運(yùn)行時(shí)間
public class JoinDemo {
public static void main(String[] args) throws InterruptedException {
System.out.println(Thread.currentThread().getName() + ": start");
long start = System.currentTimeMillis();
Thread t = new Thread(new MyRunnable());
t.start();
//阻塞主線程直到子線程完成
t.join();
long end = System.currentTimeMillis();
System.out.println("子線程花費(fèi)的時(shí)間是:" + (end - start) + "ms");
System.out.println(Thread.currentThread().getName() + ": end");
}
static class MyRunnable implements Runnable{
@Override
public void run() {
System.out.println(Thread.currentThread().getName() + ": start");
for (int i = 0; i < 100000; i ++);
System.out.println(Thread.currentThread().getName() + ": end");
}
}
}
輸出結(jié)果:
main: start
Thread-0: start
Thread-0: end
子線程花費(fèi)的時(shí)間是:3ms
main: end
- 源碼分析
我們查看join方法的源碼如下:
public final void join() throws InterruptedException {
join(0);
}
默認(rèn)設(shè)置時(shí)長(zhǎng)為0丈探,我們繼續(xù)查看join(n)方法的源碼:
public final synchronized void join(long millis)
throws InterruptedException {
long base = System.currentTimeMillis();
long now = 0;
if (millis < 0) {
throw new IllegalArgumentException("timeout value is negative");
}
//設(shè)置時(shí)長(zhǎng)為0時(shí)录择,會(huì)一直阻塞,直到被設(shè)置的線程結(jié)束
if (millis == 0) {
while (isAlive()) {
wait(0);
}
} else {
while (isAlive()) {
long delay = millis - now;
if (delay <= 0) {
break;
}
//wait方法并不能準(zhǔn)確地等待碗降,可能會(huì)被喚醒隘竭,因此需要多次檢查判斷
wait(delay);
now = System.currentTimeMillis() - base;
}
}
}
我們可以看到它是通過wait(n)方法來保證當(dāng)前線程的等待。對(duì)于上面的例子:main線程調(diào)用t.join()時(shí)讼渊,main線程會(huì)獲得對(duì)象t的鎖动看,然后調(diào)用該對(duì)象的wait(等待時(shí)間),直到該對(duì)象喚醒main線程爪幻。
- t.join(delay)方法中菱皆,當(dāng)?shù)却龝r(shí)間達(dá)到delay時(shí)赋兵,不管子線程是否執(zhí)行完畢,主線程都會(huì)繼續(xù)執(zhí)行搔预。
public class JoinDemo {
public static void main(String[] args) throws InterruptedException {
System.out.println(Thread.currentThread().getName() + ": start");
Thread t = new Thread(new MyRunnable());
t.start();
//設(shè)置等待時(shí)間為3s
t.join(3000);
System.out.println(Thread.currentThread().getName() + ": end");
}
static class MyRunnable implements Runnable{
@Override
public void run() {
System.out.println(Thread.currentThread().getName() + ": start");
try {
Thread.sleep(5000);
} catch (InterruptedException e) {
e.printStackTrace();
}
System.out.println(Thread.currentThread().getName() + ": end");
}
}
}
運(yùn)行結(jié)果:
main: start
Thread-0: start
main: end
Thread-0: end
如上霹期,子線程運(yùn)行的時(shí)間為5s,而主線程設(shè)置的等待時(shí)間為3s拯田,因此當(dāng)?shù)却龝r(shí)間達(dá)到后历造,主線程會(huì)立即執(zhí)行。
Yield方法
定義:使當(dāng)前線程從執(zhí)行狀態(tài)(運(yùn)行狀態(tài))變?yōu)榭蓤?zhí)行狀態(tài)(就緒狀態(tài))船庇。
樣例
public class YieldTest {
public static void main(String[] args) throws InterruptedException {
System.out.println(Thread.currentThread().getName() + ": start");
long start = System.currentTimeMillis();
Thread t = new Thread(new MyRunnable());
t.start();
//讓出CPU給子線程執(zhí)行任務(wù)吭产,直到結(jié)束
while (Thread.activeCount() > 1)
Thread.yield();
long end = System.currentTimeMillis();
System.out.println("子線程花費(fèi)的時(shí)間是:" + (end - start) + "ms");
System.out.println(Thread.currentThread().getName() + ": end");
}
static class MyRunnable implements Runnable{
@Override
public void run() {
System.out.println(Thread.currentThread().getName() + ": start");
for (int i = 0; i < 100000; i ++);
System.out.println(Thread.currentThread().getName() + ": end");
}
}
}
輸出結(jié)果:
main: start
Thread-0: start
Thread-0: end
子線程花費(fèi)的時(shí)間是:3ms
main: end
- yield方法的操作流程:
先檢測(cè)當(dāng)前是否有相同優(yōu)先級(jí)的線程處于同可運(yùn)行狀態(tài),如有鸭轮,則把CPU的占有權(quán)交給次線程臣淤,否則繼續(xù)運(yùn)行原來的線程,所以yield()方法稱為“退讓”窃爷,它把運(yùn)行機(jī)會(huì)讓給了同等級(jí)的其他線程邑蒋。
- yield方法與sleep方法的區(qū)別
- sleep方法使當(dāng)前運(yùn)行中的線程睡眠一段時(shí)間,進(jìn)入不可運(yùn)行狀態(tài)按厘,這段時(shí)間的長(zhǎng)短由程序設(shè)定医吊,yield方法使當(dāng)前線程讓出CPU占有權(quán),但讓出的時(shí)間是不可設(shè)定的逮京。
- sleep 方法允許較低優(yōu)先級(jí)的線程獲得運(yùn)行機(jī)會(huì)卿堂,但yield()方法執(zhí)行時(shí),當(dāng)前線程仍處在可運(yùn)行狀態(tài)懒棉,所以不可能讓出較低優(yōu)先級(jí)的線程此時(shí)獲取CPU占有權(quán)草描。