一养筒、一個例子
private static void joinTest(List<String> roles) {
Vector<Thread> vector = new Vector<>(10);
for (int i=0;i<10;i++){
MyThread myThread = new MyThread("id"+i,"name"+i,roles);
Thread t = new Thread(myThread);
vector.add(t);
t.start();
}
for (Thread thread : vector){
try {
thread.join();
} catch (InterruptedException e) {
e.printStackTrace();
}
}
}
public class MyThread implements Runnable{
private String id;
private String name;
private List<String> roles;
public MyThread(String id,String name,List<String> roles){
this.name = name;
this.id = id;
this.roles = roles;
}
@Override
public void run() {
try {
Thread.sleep(1000);
} catch (InterruptedException e) {
e.printStackTrace();
}
roles.add(name);
System.out.println("執(zhí)行了-id:"+id+"name:"+name+"roles size:"+roles.size());
}
public String getId() {
return id;
}
public void setId(String id) {
this.id = id;
}
public String getName() {
return name;
}
public void setName(String name) {
this.name = name;
}
public List<String> getRoles() {
return roles;
}
public void setRoles(List<String> roles) {
this.roles = roles;
}
}
輸出結(jié)果：
開始：1515250426924
執(zhí)行了-id:id2name:name2roles size:1
執(zhí)行了-id:id1name:name1roles size:3
執(zhí)行了-id:id0name:name0roles size:2
執(zhí)行了-id:id4name:name4roles size:4
執(zhí)行了-id:id6name:name6roles size:7
執(zhí)行了-id:id7name:name7roles size:8
執(zhí)行了-id:id8name:name8roles size:9
執(zhí)行了-id:id5name:name5roles size:6
執(zhí)行了-id:id3name:name3roles size:5
執(zhí)行了-id:id9name:name9roles size:10
[name2, name0, name1, name4, name3, name5, name6, name7, name8, name9]
結(jié)束：1515250427932耗時：1008roles size:10
可以看出斑鸦，上面每個線程執(zhí)行1000ms,總時長1008ms.
二问慎、源碼分析
/**
- Waits for this thread to die.
- <p> An invocation of this method behaves in exactly the same
- way as the invocation
- <blockquote>
- {@linkplain #join(long) join}{@code (0)}
- </blockquote>
- @throws InterruptedException
if any thread has interrupted the current thread. The<i>interrupted status</i> of the current thread iscleared when this exception is thrown.
*/
public final void join() throws InterruptedException {
join(0);
}
這里看清楚一點：join傳參為0。
然后進入join方法：
/**
- Waits at most {@code millis} milliseconds for this thread to
- die. A timeout of {@code 0} means to wait forever.
- <p> This implementation uses a loop of {@code this.wait} calls
- conditioned on {@code this.isAlive}. As a thread terminates the
- {@code this.notifyAll} method is invoked. It is recommended that
- applications not use {@code wait}, {@code notify}, or
- {@code notifyAll} on {@code Thread} instances.
- @param millis
the time to wait in milliseconds- @throws IllegalArgumentException
if the value of {@code millis} is negative- @throws InterruptedException
if any thread has interrupted the current thread. The<i>interrupted status</i> of the current thread iscleared when this exception is thrown.
*/
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");
}
if (millis == 0) {
while (isAlive()) {
wait(0);
}
} else {
while (isAlive()) {
long delay = millis - now;
if (delay <= 0) {
break;
}
wait(delay);
now = System.currentTimeMillis() - base;
}
}
}
從源碼可以看出赴背，如果線程被生成了，但還未被起動，isAlive()將返回false楞遏，調(diào)用它的join()方法是沒有作用的茬暇，將直接繼續(xù)向下執(zhí)行wait。而wait方法中有參數(shù)寡喝，也就是不用喚醒誰糙俗，只是不再執(zhí)行wait，向下繼續(xù)執(zhí)行而已预鬓。
三巧骚、join總結(jié)
join方法的原理就是調(diào)用相應(yīng)線程的wait方法進行等待操作的，例如A線程中調(diào)用了B線程的join方法格二，則相當(dāng)于在A線程中調(diào)用了B線程的wait方法劈彪，當(dāng)B線程執(zhí)行完（或者到達等待時間），B線程會自動調(diào)用自身的notifyAll方法喚醒A線程顶猜，從而達到同步的目的沧奴。
