目录
线程间通信
多线程编程步骤
一个加减实例 & 虚假唤醒问题
Lock接口实现
lock.newCondition设置等待条件
线程间定制化通信
线程间通信案例
设置标志位
线程间通信
多线程编程步骤
- 创建资源类，编写属性和操作方法
在资源中的操作方法
- 判断
干活
通知
创建多个线程，调用资源类的操作方法
使用while循环进行条件判断，防止虚假唤醒问题
一个加减实例 & 虚假唤醒问题
实现对一个初始值0进行轮流加减操作
public class AddAndSub {
private int num;
public int getNum() {
return num;
}
public AddAndSub() {
num = 0;
}
public synchronized void add() {
// 判断
while(num != 0) {
try {
this.wait();
} catch (InterruptedException e) {
e.printStackTrace();
}
}
// 业务逻辑
num++;
System.out.println(Thread.currentThread().getName() + " + 1");
// 通知
notifyAll();
}
public synchronized void sub() {
while(num != 1) {
try {
this.wait();
} catch (InterruptedException e) {
e.printStackTrace();
}
}
num--;
System.out.println(Thread.currentThread().getName() + " - 1");
notifyAll();
}
}
class RunAddAndSub {
public static void main(String[] args) {
AddAndSub addAndSub = new AddAndSub();
Runnable r1 = () -> {
for(int i = 0; i < 10; i++) {
addAndSub.add();
}
};
Runnable r2 = () -> {
for(int i = 0; i < 10; i++) {
addAndSub.sub();
}
};
new Thread(r1, "A").start();
new Thread(r2, "B").start();
System.out.println(addAndSub.getNum());
}
}
为什么要在条件判断使用while？
这是因为比如add方法A线程+1notifyAll后，可能是处于等待状态的C线程抢到了锁，但是这时候num并不是0，所以要使用while在线程唤醒后再次判断。这种问题称为虚假唤醒问题，
Lock接口实现
lock.newCondition设置等待条件
class Share {
private int num = 0;
private final Lock lock = new ReentrantLock();
private Condition condition = lock.newCondition();
public void incr() {
lock.lock();
while(num != 0) {
try {
condition.await();
} catch (InterruptedException e) {
e.printStackTrace();
}
}
num++;
System.out.println(Thread.currentThread().getName() + " + 1");
condition.signalAll();
lock.unlock();
}
public void decr() {
lock.lock();
while(num == 0) {
try {
condition.await();
} catch (InterruptedException e) {
e.printStackTrace();
}
}
num--;
System.out.println(Thread.currentThread().getName() + " - 1");
condition.signalAll();
lock.unlock();
}
}
public class TheadDemo {
public static void main(String[] args) {
var share = new Share();
Runnable r1 = () -> {
for(int i = 0; i < 10; i++) {
share.incr();
}
};
Runnable r2 = () -> {
for(int i = 0; i < 10; i++) {
share.decr();
}
};;
new Thread(r1, "A").start();
new Thread(r2, "B").start();
new Thread(r1, "C").start();
new Thread(r2, "D").start();
}
}
注意condition.signalAll()要写在unlock之前
线程间定制化通信
如上面的四个线程执行的时候，只是+-线程的顺序执行，执行相同方法的线程是没有顺序的，规定相同方法的线程的执行顺序，即是线程的定制化通信。
线程间通信案例
启动三个线程，要求如下：
AA打印5次，BB打印10次，CC打印15次
如此进行10轮
设置标志位
当标志位为1，AA执行，当标志位为2，BB执行，当标志位为3，CC执行
class Print {
private int flag = 1;
public synchronized void print5(int num) throws InterruptedException {
while (flag != 1) {
this.wait();
}
for (int i = 0; i < 5; i++) {
System.out.println(Thread.currentThread().getName() + ":::" + i + " ,loop" + num);
}
flag = 2;
this.notifyAll();
}
public synchronized void print10(int num) throws InterruptedException {
while (flag != 2) {
this.wait();
}
for (int i = 0; i < 10; i++) {
System.out.println(Thread.currentThread().getName() + ":::" + i + " ,loop" + num);
}
flag = 3;
this.notifyAll();
}
public synchronized void print15(int num) throws InterruptedException {
while (flag != 3) {
this.wait();
}
for (int i = 0; i < 15; i++) {
System.out.println(Thread.currentThread().getName() + ":::" + i + " ,loop" + num);
}
flag = 1;
this.notifyAll();
}
}
public class ThreadDemo2 {
public static void main(String[] args) {
Print print = new Print();
new Thread(new Runnable() {
@Override
public void run() {
for (int i = 0; i < 10; i++) {
try {
print.print5(i);
} catch (InterruptedException e) {
e.printStackTrace();
}
}
}
}, "AA").start();
new Thread(new Runnable() {
@Override
public void run() {
for (int i = 0; i < 10; i++) {
try {
print.print10(i);
} catch (InterruptedException e) {
e.printStackTrace();
}
}
}
}, "BB").start();
new Thread(new Runnable() {
@Override
public void run() {
for (int i = 0; i < 10; i++) {
try {
print.print15(i);
} catch (InterruptedException e) {
e.printStackTrace();
}
}
}
}, "CC").start();
}
}
