您可能已经注意到 Object 类具有三个 final 方法,分别称为 wait、notify 和 notifyAll。这些方法用于线程间通信。Java 5 引入了执行器框架,它为您处理线程间通信,并在内部使用 wait、notify 和 notifyAll,但您仍然需要基本了解这些方法以及 java 中使用 wait、notify 和 notifyAll 的线程间通信如何工作。
wait 、 notify 和 notifyAll 方法用于允许线程通过访问公共对象来相互通信,或者换句话说,可以将 Object 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,否则会抛出 java.lang.IllegalMonitorStateException。
同步块的一些背景:
让我们对这些方法有一些简要的了解:
当您在对象上调用 wait 方法时,它会告诉线程放弃锁定并进入睡眠状态,除非并且直到某个其他线程进入同一监视器并在其上调用 notify 或 notifyAll 方法。
当您在对象上调用 notify 方法时,它会唤醒等待该对象的线程之一。因此,如果多个线程正在等待一个对象,它将唤醒其中一个。现在你一定想知道它会唤醒哪一个。它实际上取决于操作系统的实现。
notifyAll 将唤醒等待该对象的所有线程,不像 notify 只唤醒其中一个。哪个将首先唤醒取决于线程优先级和操作系统实现。
让我们借助示例来理解它:
1. 创建一个名为 Book.java 的类: 这是一个 java bean 类,线程将在该类上执行并调用 wait 和 notify 方法。
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
这个线程会一直等到其他线程调用notify方法,然后才会完成它的处理。它将首先锁定 book 对象,并从同步块中调用。因此在此示例中,它将等待 BookWriter 完成这本书。
ackage 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籍对象的线程(在通知的情况下)。它不会在调用 notify 后立即放弃锁,它首先完成其同步块。所以在这个例子中,BookWriter 将完成这本书并将其通知给 BookReaders。
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。
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(); } }
如果是 notify(): 当你运行上面的程序时,你会得到以下输出:
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
所以在这里,两个 BookReader 线程(arpit 和 john)正在等待 book 完成,所以他们调用 book.wait()。一旦 BookWriter 完成它,它调用 book.notify() 并且 arpit 线程启动并完成它的处理。
在 notifyAll() 的情况下: 让我们将 BookWriter 类更改为调用 book.notifyAll()。
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"); } } }
当你运行上面的程序时,你会得到以下输出:
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
在 notifyAll() 的情况下,它会通知所有等待该对象的线程。
原文链接:https://codingdict.com/