文章目錄
Volatile 內存共享
基於 volatile 關鍵字來實現線程間相互通信是使用共享內存的思想,上一章有過介紹,想具體瞭解的碼友可以去看一下,Java併發編程3–認識Volatile和JMM
public class VolatileThread implements Runnable {
private static volatile Boolean flag = true;
@Override
public void run() {
while (flag) {
System.out.println(Thread.currentThread().getName() + " - 執行");
}
System.out.println("線程結束");
}
public static void main(String[] args) {
Thread volatileThread = new Thread(new VolatileThread());
volatileThread.start();
try {
Thread.sleep(5);
flag = false;
} catch (InterruptedException e) {
e.printStackTrace();
}
}
}
wait/notify 等待
看個案例,比較經典的,兩個線程,一個線程打印1-25,另一個線程打印字母A~Z,打印順序爲1A2B…25Z,要求使用線程間的通信。
這是利用wait/notify 最基本的線程通信,threadFlag是打印數字還是字母的標識,
import java.util.ArrayList;
import java.util.Arrays;
import java.util.List;
public class MethodOne {
static class ThreadToGo {
boolean threadFlag = true;
}
public static void main(String args[]) throws Exception {
final ThreadToGo threadToGo = new ThreadToGo();
List<String> list = new ArrayList<>();
List<String> listSZ = Arrays.asList("1","2","3","4","5","6");
List<String> listZM = Arrays.asList("A","B","C","D","E","F");
Thread thread1 = new Thread(()->{
synchronized (threadToGo) {
for (int i = 0; i < listZM.size(); i++) {
if(threadToGo.threadFlag == true){
try {
threadToGo.wait();
} catch (InterruptedException e) {
}
}
threadToGo.threadFlag = true;
list.add(listZM.get(i));
threadToGo.notify();
}
}
});
Thread thread2 = new Thread(()->{
synchronized (threadToGo) {
for (int i = 0; i < listSZ.size(); i++) {
if(threadToGo.threadFlag != true){
try {
threadToGo.wait();
} catch (InterruptedException e) {
}
}
threadToGo.threadFlag = false;
list.add(listSZ.get(i));
threadToGo.notify();
}
}
});
thread1.start();
thread2.start();
Thread.sleep(3000);
System.out.println(list.toString());
}
}
使用JUC工具類CountDownLatch
CountDownLatch是一個同步工具類,用來協調多個線程之間的同步,或者說起到線程之間的通信(而不是用作互斥的作用)。
CountDownLatch典型用法:
- 1、某一線程在開始運行前等待n個線程執行完畢。將CountDownLatch的計數器初始化爲new CountDownLatch(n),每當一個任務線程執行完畢,就將計數器減1 countdownLatch.countDown(),當計數器的值變爲0時,在CountDownLatch上await()的線程就會被喚醒。一個典型應用場景就是啓動一個服務時,主線程需要等待多個組件加載完畢,之後再繼續執行。
- 2、實現多個線程開始執行任務的最大並行性。注意是並行性,不是併發,強調的是多個線程在某一時刻同時開始執行。類似於賽跑,將多個線程放到起點,等待發令槍響,然後同時開跑。做法是初始化一個共享的CountDownLatch(1),將其計算器初始化爲1,多個線程在開始執行任務前首先countdownlatch.await(),當主線程調用countDown()時,計數器變爲0,多個線程同時被喚醒。
注意: CountDownLatch是一次性的,計算器的值只能在構造方法中初始化一次,之後沒有任何機制再次對其設置值,當CountDownLatch使用完畢後,它不能再次被使用。所以在線程通信中使用比較侷限。
使用 ReentrantLock 結合 Condition
打印1A2B…25Z的案例用Condition實現,
import java.util.ArrayList;
import java.util.Arrays;
import java.util.List;
import java.util.concurrent.locks.Condition;
import java.util.concurrent.locks.ReentrantLock;
public class MethodOne {
static class ThreadToGo {
boolean threadFlag = true;
}
public static void main(String args[]) throws Exception {
final ThreadToGo threadToGo = new ThreadToGo();
List<String> list = new ArrayList<>();
ReentrantLock lock = new ReentrantLock();
Condition condition = lock.newCondition();
List<String> listSZ = Arrays.asList("1","2","3","4","5","6");
List<String> listZM = Arrays.asList("A","B","C","D","E","F");
Thread thread1 = new Thread(()->{
lock.lock();
for (int i = 0; i < listZM.size(); i++) {
if(threadToGo.threadFlag == true){
try {
condition.await();
} catch (InterruptedException e) {
// TODO Auto-generated catch block
e.printStackTrace();
}
}
threadToGo.threadFlag = true;
list.add(listZM.get(i));
condition.signal();
}
lock.unlock();
});
Thread thread2 = new Thread(()->{
lock.lock();
for (int i = 0; i < listSZ.size(); i++) {
if(threadToGo.threadFlag != true){
try {
condition.await();
} catch (InterruptedException e) {
e.printStackTrace();
}
}
threadToGo.threadFlag = false;
list.add(listSZ.get(i));
condition.signal();
}
lock.unlock();
});
thread1.start();
thread2.start();
Thread.sleep(3000);
System.out.println(list.toString());
}
}
基本LockSupport實現線程間的阻塞和喚醒
LockSupport 是一種非常靈活的實現線程間阻塞和喚醒的工具,使用它不用關注是等待線程先進行還是喚醒線程先運行,但是得知道線程的名字。
- LockSupport.park():可以用來阻塞當前線程,park是停車的意思,把運行的線程比作行駛的車輛,線程阻塞則相當於汽車停車,相當直觀。
- LockSupport.park(Object blocker),指定線程阻塞的對象blocker,該對象主要用來排查問題。
- LockSupport.unpark(Thread thread)用來喚醒線程,因爲需要線程作參數,所以可以指定線程進行喚醒。
public class LockSupportTest {
public static void main(String[] args) {
Thread parkThread = new Thread(()->{
System.out.println("開始線程阻塞");
LockSupport.park();
System.out.println("結束線程阻塞");
});
parkThread.start();
System.out.println("開始線程喚醒");
LockSupport.unpark(parkThread);
System.out.println("結束線程喚醒");
}
}