本文內(nèi)容提要：wait()呻粹、notify()、join()昼扛、sleep()寸齐、yield()、interrupt()、ThreadLocal渺鹦、InheritThreadLocal扰法、TransmittableThreadLocal。
Thread的生命周期
Thread的生命周期分為初始化毅厚，就緒塞颁，運(yùn)行，阻塞吸耿，終止祠锣，其中只有運(yùn)行狀態(tài)的線程擁有CPU資源的時(shí)間片。
Object-線程的wait()和notify()
? 線程的等待和通知方法放在Object類里而非Thread類珍语，對于wait()方法來說锤岸，必須在調(diào)用之前獲取對應(yīng)實(shí)例的監(jiān)視器鎖，否則會(huì)拋出IllegalMonitorStateException板乙。而通常是偷，鎖資源可以是任意對象，把wait()募逞、notify()蛋铆、notifyAll()方法放在Obejct方法里，符合Java把所有類都會(huì)使用的方法定義在Object類的思想放接。
? 注意：正如前文所提：調(diào)用wait()之前刺啦，必須在調(diào)用之前獲取對應(yīng)實(shí)例的監(jiān)視器鎖。
private static void interruptTest() throws InterruptedException {
Integer obj = 1;
Thread a = new Thread(new Runnable() {
@Override
public void run() {
System.out.println("this is begining");
try {
synchronized (obj) {
obj.wait();
}
} catch (InterruptedException e) {
e.printStackTrace();
}
System.out.println("this is ending");
}
});
a.start();
a.join();
}
? 當(dāng)線程調(diào)用wait方法后纠脾，會(huì)釋放鎖資源玛瘸，并進(jìn)入阻塞狀態(tài)。等待其它線程調(diào)用notify()方法苟蹈、或者notifyAll()方法喚醒糊渊，或者interrupt中斷和wait(time)調(diào)用后等待超時(shí)的虛假喚醒。當(dāng)調(diào)用notify()函數(shù)慧脱，且對于鎖對象obj渺绒，存在多個(gè)線程處于阻塞狀態(tài)，會(huì)隨機(jī)選一個(gè)進(jìn)行喚醒菱鸥。而notifyAll()則會(huì)喚醒obj下所有阻塞的對象宗兼。注意：喚醒并不代表立刻執(zhí)行，而是競爭鎖氮采，競爭到鎖后才會(huì)到就緒狀態(tài)殷绍，只有等到競爭到CPU資源也就是時(shí)間片后才變成運(yùn)行狀態(tài)繼續(xù)執(zhí)行。
? 上述的運(yùn)行->阻塞->就緒->執(zhí)行的狀態(tài)轉(zhuǎn)換涉及到一個(gè)細(xì)節(jié)鹊漠，就是線程如何知道再次執(zhí)行時(shí)從哪里開始繼續(xù)往下執(zhí)行篡帕，因此會(huì)在阻塞時(shí)殖侵，或者說進(jìn)行時(shí)間片切換時(shí)，記錄當(dāng)前執(zhí)行地址镰烧，這里用到的是線程私有的程序計(jì)數(shù)器。
Thread里的方法
等待線程終止的join()方法
? 有時(shí)候存在這樣的需求楞陷，主線程開啟n個(gè)子線程怔鳖，并希望在所有子線程結(jié)束后在進(jìn)行一些邏輯操作。這時(shí)候就需要用到j(luò)oin()方法固蛾。
public static void main(String[] args) throws InterruptedException {
final Thread mainThread = Thread.currentThread();
Thread a = new Thread(() -> {
try {
Thread.sleep(100);
} catch (InterruptedException e) {
e.printStackTrace();
}
System.out.println("subThread is over");
});
a.start();
a.join();
System.out.println("main is over");
}
輸出結(jié)果為：
subThread is over
main is over
主線程在調(diào)用了a線程后進(jìn)入阻塞狀態(tài)结执，這時(shí)可以通過interrupt()方法中斷阻塞狀態(tài)。
public static void main(String[] args) throws InterruptedException {
final Thread mainThread = Thread.currentThread();
Thread a = new Thread(() -> {
while (true) {
}
});
Thread b = new Thread(() -> {
try {
Thread.sleep(50);
} catch (InterruptedException e) {
e.printStackTrace();
}
mainThread.interrupt();
});
b.start();
a.start();
try {
a.join();
} catch (InterruptedException e) {
System.out.println("main is interrupted");
}
System.out.println("main is over");
}
輸出結(jié)果為：
main is interrupted
main is over
讓線程睡眠的sleep()方法
sleep()方法會(huì)讓當(dāng)前線程進(jìn)入阻塞狀態(tài)艾凯，但不會(huì)釋放鎖資源献幔。
public static void main(String[] args) throws InterruptedException {
final Thread mainThread = Thread.currentThread();
Integer lock1 = 1, lock2 = 1;
Thread a = new Thread(() -> {
synchronized (lock1) {
System.out.println("a get lock1");
try {
Thread.sleep(1000);
} catch (InterruptedException e) {
e.printStackTrace();
}
synchronized (lock2) {
System.out.println("a get lock2");
}
}
});
Thread b = new Thread(() -> {
synchronized (lock1) {
System.out.println("b get lock1");
synchronized (lock2) {
System.out.println("b get lock2");
}
}
});
a.start();
b.start();
a.join();b.join();
System.out.println("main is over");
}
? 在上面的例子中，由于a線程先獲取到鎖資源lock1趾诗，即使a調(diào)用sleep()方法進(jìn)入阻塞狀態(tài)蜡感，b線程仍然無法獲取到鎖資源lock1（即lock1在a線程sleep()之后并沒有被釋放）。注意：sleep()入?yún)⒉荒転樨?fù)數(shù)恃泪，會(huì)拋出異常郑兴。
讓出CPU時(shí)間片的yield()方法
? yield()方法調(diào)用后會(huì)暗示線程調(diào)度器希望讓出當(dāng)前線程所占的時(shí)間片，但是線程調(diào)度器可以無條件忽略這個(gè)暗示贝乎。如果yield()方法成功讓出CPU時(shí)間片情连，就會(huì)進(jìn)入就緒狀態(tài)，等待重新競爭到時(shí)間片繼續(xù)執(zhí)行览效。所以却舀，存在這樣的情況線程A在調(diào)用yield()方法后，通過競爭在下一輪線程調(diào)度中再次獲取到了時(shí)間片锤灿。同樣的挽拔，yield()方法不會(huì)讓出鎖資源，下面的demo可以證明即使a線程yield()衡招，b線程獲取到時(shí)間片開始執(zhí)行篱昔，仍然無法獲取到lock1資源，所以輸出結(jié)果仍然是先執(zhí)行完a線程始腾。
public static void main(String[] args) throws InterruptedException {
final Thread mainThread = Thread.currentThread();
Integer lock1 = 1, lock2 = 1;
Thread a = new Thread(() -> {
synchronized (lock1) {
System.out.println("a get lock1");
Thread.yield();
synchronized (lock2) {
System.out.println("a get lock2");
}
}
});
Thread b = new Thread(() -> {
System.out.println("b get cpu!");
synchronized (lock1) {
System.out.println("b get lock1");
synchronized (lock2) {
System.out.println("b get lock2");
}
}
});
a.start();
b.start();
a.join();
b.join();
System.out.println("main is over");
}
輸出結(jié)果為：
a get lock1
b get cpu!
a get lock2
b get lock1
b get lock2
main is over
如果將yield()替換為wait()州刽，a線程進(jìn)入阻塞狀態(tài)后，釋放資源浪箭，b線程成功獲取到lock1鎖資源穗椅，輸出結(jié)果證明他們的差異。
public static void main(String[] args) throws InterruptedException {
final Thread mainThread = Thread.currentThread();
Integer lock1 = 1, lock2 = 1;
Thread a = new Thread(() -> {
synchronized (lock1) {
System.out.println("a get lock1");
try {
lock1.wait();
} catch (InterruptedException e) {
System.out.println("a is interrupted");
}
synchronized (lock2) {
System.out.println("a get lock2");
}
}
});
Thread b = new Thread(() -> {
System.out.println("b get cpu!");
synchronized (lock1) {
System.out.println("b get lock1");
synchronized (lock2) {
System.out.println("b get lock2");
}
a.interrupt();
}
});
a.start();
b.start();
a.join();
b.join();
System.out.println("main is over");
}
輸出結(jié)果：
a get lock1
b get cpu!
b get lock1
b get lock2
a is interrupted
a get lock2
main is over
設(shè)置中斷標(biāo)志的interrupt()方法
? 前文的最佳配角interrupt()方法奶栖，并非暴力地直接中斷對應(yīng)的線程匹表，而是對對應(yīng)的線程設(shè)置中斷標(biāo)志门坷。
// 檢測當(dāng)前實(shí)例線程是否被中斷，中斷true袍镀，否則false
private native boolean isInterrupted(boolean ClearInterrupted);
// 檢測當(dāng)前線程是否被中斷默蚌，如果發(fā)現(xiàn)線程被中斷，會(huì)清除中斷標(biāo)志苇羡，返回true绸吸。否則返回false
private native boolean interrupted(){
return currentThread().isInterrupted(true);
}
// 設(shè)置中斷標(biāo)志位true
public void interrupt();
interrupted()檢測的是當(dāng)前線程
這里要注意的是interrupted()檢測的是當(dāng)前線程，跟句柄無關(guān)设江。如下面的demo：
public static void main(String[] args) throws InterruptedException {
final Thread mainThread = Thread.currentThread();
Thread a = new Thread(() -> {
while (true) {
}
});
a.start();
a.interrupt();
System.out.println();
System.out.println("is interrupted :" + a.isInterrupted()); // 1
System.out.println("is interrupted :" + a.interrupted()); // 2
System.out.println("is interrupted :" + a.isInterrupted()); // 3
}
輸出結(jié)果：
true
false
true
2處雖然句柄為a線程锦茁，但是正如前文所述，在interrupted()方法中會(huì)調(diào)用Thread.getCurrentThread()方法獲取當(dāng)前線程叉存，獲取到線程為主線程码俩，而主線程并未被中斷，所以輸出false歼捏。
interrupt() 只是設(shè)置中斷標(biāo)志稿存，并非直接中斷
public static void main(String[] args) throws InterruptedException {
final Thread mainThread = Thread.currentThread();
Thread a = new Thread(() -> {
while (true) {
System.out.println("a is working");
}
});
a.start();
a.interrupt();
a.join();
}
輸出結(jié)果：
a is working
a is working
a is working
a is working
a is working
...
可以發(fā)現(xiàn)如果a在內(nèi)部沒有調(diào)用wait、sleep等方法進(jìn)入阻塞狀態(tài)甫菠，就不會(huì)被中斷挠铲。
ThreadLocal — 你不得不知道的坑
? ThreadLocal只能在保證當(dāng)前線程可以獲取到對應(yīng)的變量。
? 考慮到存在這樣的情況寂诱，主線程在ThreadLocal中放了參數(shù)拂苹，并啟用了多個(gè)子線程進(jìn)行工作，同時(shí)子線程需要用到前面主線程在ThreadLocal中放置的參數(shù)痰洒。這時(shí)候考慮到用InheritThreadLocal瓢棒，在Thread.init()方法源碼中可以看到，當(dāng)線程初始化時(shí)丘喻，InheritThreadLocal中存放的參數(shù)會(huì)被復(fù)制到子線程的InheritThreadLocal中脯宿。
private void init(ThreadGroup g, Runnable target, String name,
long stackSize, AccessControlContext acc,
boolean inheritThreadLocals) {
if (name == null) {
throw new NullPointerException("name cannot be null");
}
...
...
if (inheritThreadLocals && parent.inheritableThreadLocals != null)
this.inheritableThreadLocals =
ThreadLocal.createInheritedMap(parent.inheritableThreadLocals);
/* Stash the specified stack size in case the VM cares */
this.stackSize = stackSize;
/* Set thread ID */
tid = nextThreadID();
}
? 那么是否InheritThreadLocal就已經(jīng)能解決多線程問題了呢？答案是并不能泉粉。因?yàn)槲覀冎涝诰€程池的使用中连霉，為了減少線程初始化和銷毀的性能消耗，提出了線程復(fù)用的概念嗡靡。對于核心線程來說跺撼，一旦被初始化后，就不會(huì)被銷毀讨彼。對于InheritThreadLocal而言歉井，其變量的傳遞主要依賴于Thread.init()方法中進(jìn)行參數(shù)復(fù)制傳遞。
? 所以當(dāng)使用線程池時(shí)哈误，會(huì)發(fā)現(xiàn)每個(gè)線程的InheritThreadLocal中的參數(shù)哩至，一旦被賦值后就不會(huì)再更新躏嚎，也就失去了它的正確性，可以理解為是非線程安全的菩貌。這時(shí)候可以考慮使用TransmittableThreadLocal來解決卢佣，具體可見TTL項(xiàng)目的官網(wǎng)說明（如傳遞鏈路id等）。
