Java 中 wait()和 notify()的区别
等待()和通知()是对象类的方法。他们被介绍与轮询分道扬镳，轮询是反复检查要满足的条件的过程。轮询会相当浪费 CPU 资源，因此不是首选。
wait()有 3 个变体:
1。不接受任何参数的基本版本
public final void wait()
// will cause thread to wait till notify is called
2。采用单个超时参数的版本
public final void wait(long timeout)
// will cause thread to wait either till notify is called or
// till timeout (One which occurs earlier)
3。版本采用超时参数和纳秒参数以获得额外的精度
public final void wait(long timeout,
int nanoseconds)
notify()有 1 个签名
public final void notify()
// Function does not take any argument
下面是 wait()和 notify()方法的演示:
Java 语言(一种计算机语言，尤用于创建网站)
// Java Program to demonstrate usage of wait() and notify()
class demo {
// variable to check if part1 has returned
// volatile used to prevent threads from
// storing local copies of variable
volatile boolean part1done = false;
// method synchronized on this
// i.e. current object of demo
synchronized void part1()
{
System.out.println("Welcome to India");
part1done = true;
System.out.println(
"Thread t1 about to surrender lock");
// notify the waiting thread, if any
notify();
}
// method synchronized on this
// i.e. current object of demo
synchronized void part2()
{
// loop to prevent spurious wake-up
while (!part1done) {
try {
System.out.println("Thread t2 waiting");
// wait till notify is called
wait();
System.out.println(
"Thread t2 running again");
}
catch (Exception e) {
System.out.println(e.getClass());
}
}
System.out.println("Do visit Taj Mahal");
}
}
public class Main {
public static void main(String[] args)
{
// Make an instance of demo class
demo obj = new demo();
// Thread t1 will call part1()
Thread t1 = new Thread(new Runnable() {
public void run() { obj.part1(); }
});
// Thread t2 will call part2()
Thread t2 = new Thread(new Runnable() {
public void run() { obj.part2(); }
});
// Start t2 and then t1
t2.start();
t1.start();
}
}
Output
Thread t2 waiting
Welcome to India
Thread t1 about to surrender lock
Thread t2 running again
Do visit Taj Mahal
各种异常
A .等待()
- 必须在 try-catch 块中包含 wait()，因为如果处于等待状态的线程被中断，它将引发中断异常。
- 如果超时值为负或者纳秒值不在 0 到 9，99，999 的范围内，等待外壳参数的另外两个变体将引发 IllegalArgumentException。
下面是异常处理的实现。
Java 语言(一种计算机语言，尤用于创建网站)
// Program demonstrating occurrence of InterruptedException
class demo {
volatile boolean part1done = false;
synchronized void part1()
{
System.out.println("Welcome to India");
part1done = true;
// notify() has been commented, waiting
// thread remains waiting forever notify();
}
synchronized void part2()
{
while (!part1done) {
try {
wait();
}
catch (Exception e) {
System.out.println("Exception : "
+ e.getClass());
// quit program after exception is thrown
System.exit(-1);
}
}
System.out.println("Do visit Taj Mahal");
}
}
public class Main {
public static void main(String[] args)
{
// Make an instance of demo class
demo obj = new demo();
// Thread t1 will call part1()
Thread t1 = new Thread(new Runnable() {
public void run() { obj.part1(); }
});
// Thread t2 will call part2()
Thread t2 = new Thread(new Runnable() {
public void run() { obj.part2(); }
});
// Start t2 and then t1
t2.start();
t1.start();
// This is a counter which will
// interrupt Thread t2 after 3 seconds
long startTime = System.currentTimeMillis();
while (true) {
if (System.currentTimeMillis() - startTime
> 3000)
t2.interrupt();
}
}
}
Output
Welcome to India
Exception : class java.lang.InterruptedException
B. notify()
与 wait()不同，notify 方法不会引发中断异常，因此不强制将其包含在 try-catch 块中
注:
- wait()和 notify()都有抛出 IllegalMonitorStateException 的倾向
- 当线程持有对象 A 的监视器锁并试图调用对象 b 上的等待或通知时，就会发生这种情况。
- 在前面的所有例子中，方法都是在“this”上同步的，即用于调用这些方法的对象(obj)。此外，wait() & notify()被调用为 this.wait()和 this.notify()(这种用法是多余的)。因此，不存在任何问题。
- 在下面的例子中，方法 part1 和 part2 现在在一个整数对象上同步，但是 wait() & notify()仍然在调用这些方法(obj)的对象上被调用。
- 这将导致非法监控状态异常
下面是异常处理的实现。
Java 语言(一种计算机语言，尤用于创建网站)
// Program to demonstrate IllegalMonitorStateException
class demo {
volatile boolean part1done = false;
// Made an Integer object a
// and set it randomly to 5
Integer a = 5;
void part1()
{
// Synchronized code on a
synchronized (a)
{
System.out.println("Welcome to India");
part1done = true;
// calling this.notify()
notify();
}
}
void part2()
{
// Synchronized code on a
synchronized (a)
{
while (!part1done) {
try {
// calling this.wait()
wait();
}
catch (Exception e) {
System.out.println("Exception: "+e.getClass());
System.exit(-1);
}
}
System.out.println("Do visit Taj Mahal");
}
}
}
public class Main {
public static void main(String[] args)
{
// Make an instance of demo class
demo obj = new demo();
// Thread t1 will call part1()
Thread t1 = new Thread(new Runnable() {
public void run() { obj.part1(); }
});
// Thread t2 will call part2()
Thread t2 = new Thread(new Runnable() {
public void run() { obj.part2(); }
});
// Start t2 and then t1
t2.start();
t1.start();
}
}
Output
Exception: class java.lang.IllegalMonitorStateException
要修复上述代码，只需在第 19 行用 a.notify()替换 notify()，在第 31 行用 a.wait()替换 wait()。
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