本文深入探讨了Java中Object类的wait、notify和notifyAll方法在多线程通信中的应用。这些方法需要在同步块中调用，否则会抛出异常。wait()使线程进入等待状态，直到被其他线程唤醒；notify()唤醒一个等待的线程，而notifyAll()唤醒所有等待的线程。通过示例展示了如何在实际代码中使用这些方法，以实现线程间的协调工作。
您可能已经注意到Object类具有三个最终方法，分别称为wait，notify和notifyAll。这些方法用于线程间通信。Java 5引入了执行程序框架，该执行程序框架为您处理线程间通信，并在内部使用wait，notify和notifyAll。但是您仍然需要对这些方法以及线程间通信如何在Java中使用wait，notify和notifyAll进行基本了解。
什么是wait，notify和notifyAll方法？
wait，notify和notifyAll方法用于允许线程通过访问公共对象彼此通信，或者换句话说，对象可以被视为通过这些方法进行线程间通信的媒介。这些方法需要从同步上下文中调用，否则将抛出java.lang.IllegalMonitorStateException。
同步块的一些背景：
- 同步块中一次只能进入一个线程
- 线程需要锁定对象才能进入同步块。
- 如果线程A要进入同步块，则线程A必须等待线程B释放它。
让我们对这些方法有一些简要的了解：
wait()：
当您在对象上调用wait方法时，它告诉线程放弃锁并进入睡眠状态，除非并且直到其他线程进入同一监视器并在其上调用notify或notifyAll方法。
notify()：
当您在对象上调用notify方法时，它将唤醒等待该对象的线程之一。因此，如果有多个线程在等待一个对象，它将唤醒其中之一。现在，您一定想知道它将唤醒哪一个。它实际上取决于OS的实现。
notifyAll()：
notifyAll会唤醒所有在该对象上等待的线程，而不是notify会只唤醒其中一个线程，哪个线程会首先唤醒取决于线程优先级和操作系统实现。
让我们借助示例了解它：
1.创建一个名为Book.java的类：
这是Java bean类，线程将在该Java bean类上作用并调用wait和notify方法。
|
1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
|
package org.arpit.java2blog.thread;
public class Book {
String title;
boolean isCompleted;
public Book(String title) {
super();
this.title = title;
}
public String getTitle() {
return title;
}
public void setTitle(String title) {
this.title = title;
}
public boolean isCompleted() {
return isCompleted;
}
public void setCompleted(boolean isCompleted) {
this.isCompleted = isCompleted;
}
}
|
2.创建一个名为BookReader.java的类
该线程将等到其他线程调用notify方法，然后再完成其处理。它将首先锁定书本对象，然后从同步块中调用它。因此在本示例中，它将等待BookWriter完成书本。
|
1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
|
package org.arpit.java2blog.thread;
public class BookReader implements Runnable{
Book book;
public BookReader(Book book) {
super();
this.book = book;
}
@Override
public void run() {
synchronized (book) {
System.out.println(Thread.currentThread().getName()+" is waiting for the book to be completed: "+book.getTitle());
try {
book.wait();
} catch (InterruptedException e) {
e.printStackTrace();
}
System.out.println(Thread.currentThread().getName()+": Book has been completed now!! you can read it");
}
}
}
|
3.创建一个名为BookWriter.java的类
此类将通知正在等待书本对象的线程（如果有notify的话）。一旦调用notify，它将不会放弃锁，它首先完成其同步块。因此，在此示例中，BookWriter将完成书籍并将其通知给BookReaders。
|
1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
29
30
|
package org.arpit.java2blog.thread;
public class BookWriter implements Runnable{
Book book;
public BookWriter(Book book) {
super();
this.book = book;
}
@Override
public void run() {
synchronized (book) {
System.out.println("Author is Starting book : " +book.getTitle() );
try {
Thread.sleep(1000);
} catch (InterruptedException e) {
e.printStackTrace();
}
book.setCompleted(true);
System.out.println("Book has been completed now");
book.notify();
System.out.println("notify one reader");
}
}
}
|
4.创建一个类ThreadInterCommunicationMain，java。
这是我们的主类，它将创建上述类的对象并运行它。
|
1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
29
30
31
32
33
34
35
|
package org.arpit.java2blog.thread;
public class ThreadInterCommunicationMain {
public static void main(String args[])
{
// Book object on which wait and notify method will be called
Book book=new Book("The Alchemist");
BookReader johnReader=new BookReader(book);
BookReader arpitReader=new BookReader(book);
// BookReader threads which will wait for completion of book
Thread johnThread=new Thread(johnReader,"John");
Thread arpitThread=new Thread(arpitReader,"Arpit");
arpitThread.start();
johnThread.start();
// To ensure both readers started waiting for the book
try {
Thread.sleep(3000);
} catch (InterruptedException e) {
e.printStackTrace();
}
// BookWriter thread which will notify once book get completed
BookWriter bookWriter=new BookWriter(book);
Thread bookWriterThread=new Thread(bookWriter);
bookWriterThread.start();
}
}
|
在notify()的情况下：
当您运行上述程序时，将得到以下输出：
|
1
2
3
4
5
6
7
8
|
Arpit is waiting for the book to be completed: The Alchemist
John is waiting for the book to be completed: The Alchemist
Author is Starting book : The Alchemist
Book has been completed now
notify one reader
Arpit: Book has been completed now!! you can read it
|
如果是notifyAll()：可以
将BookWriter类更改为调用book.notifyAll（）。
|
1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
29
30
31
32
33
|
package org.arpit.java2blog.thread;
public class BookWriter implements Runnable{
Book book;
public BookWriter(Book book) {
super();
this.book = book;
}
@Override
public void run() {
synchronized (book) {
System.out.println("Author is Starting book : " +book.getTitle() );
try {
Thread.sleep(1000);
} catch (InterruptedException e) {
// TODO Auto-generated catch block
e.printStackTrace();
}
book.setCompleted(true);
System.out.println("Book has been completed now");
book.notifyAll();
System.out.println("notify readers");
}
}
}
|
当您运行上述程序时，将得到以下输出：
|
1
2
3
4
5
6
7
8
9
|
Arpit is waiting for the book to be completed: The Alchemist
John is waiting for the book to be completed: The Alchemist
Author is Starting book : The Alchemist
Book has been completed now
notify readers
John: Book has been completed now!! you can read it
Arpit: Book has been completed now!! you can read it
|
如果是notifyAll（），它将通知所有正在等待该对象的线程。
扫一扫
被折叠的 条评论
为什么被折叠?
到【灌水乐园】发言
