Java中解決多執行緒資料安全問題

同步程式碼塊

基本語句

synchronized (任意物件) {

操作共享程式碼

}

程式碼示例

public class SellTicket implements Runnable {

    private int tickets = 100;

    private Object object = new Object();

    @Override

    public void run() {

        while (true) {

            synchronized (object) {

                if (tickets > 0) {

                    try {

                        Thread.sleep(100);

                    } catch (InterruptedException e) {

                        e.printStackTrace();

                    }

                    System.out.println(Thread.currentThread().getName() + "正在出售第" + tickets + "張票");

                    tickets--;

                }

            }

        }

    }

    public static void main(String[] args) {

        SellTicket sellTicket = new SellTicket();

        Thread thread1 = new Thread(sellTicket, "視窗1");

        Thread thread2 = new Thread(sellTicket, "視窗2");

        Thread thread3 = new Thread(sellTicket, "視窗3");

        thread1.start();

        thread2.start();

        thread3.start();

    }

}

優缺點：

• 解決了多執行緒的資料安全問題
• 多執行緒時，每個執行緒都會判斷同步上的鎖，耗費資源，降低了程式的執行效率

同步方法

同步方法：將synchronized關鍵字加到方法上

• 格式: 修飾符 synchronized 返回值型別 方法名(){ }
• 同步方法的鎖物件是this

同步靜態方法，就是把synchronized關鍵字加到靜態方法上

• 格式: 修飾符 static synchronized 返回值型別 方法名(){ }
• 同步靜態方法的鎖物件是 類名.class

程式碼示例

public class SellTicket implements Runnable {

//    private int tickets = 100;

    private static int tickets = 100;

    private Object object = new Object();

    private int x = 0;

    @Override

    public void run() {

        while (true) {

            if (x % 2 == 0) {

//                synchronized (object) {

//                synchronized (this) {

                synchronized (SellTicket.class) {

                    if (tickets > 0) {

                        try {

                            Thread.sleep(100);

                        } catch (InterruptedException e) {

                            e.printStackTrace();

                        }

                        System.out.println(Thread.currentThread().getName() + "正在出售第" + tickets + "張票");

                        tickets--;

                    }

                }

            } else {

//                synchronized (object) {

//                    if (tickets > 0) {

//                        try {

//                            Thread.sleep(100);

//                        } catch (InterruptedException e) {

//                            e.printStackTrace();

//                        }

//                        System.out.println(Thread.currentThread().getName() + "正在出售第" + tickets + "張票");

//                        tickets--;

//                    }

//                }

                sellTicket();

            }

            x++;

        }

    }

//    private void sellTicket(){

//        synchronized (object) {

//            if (tickets > 0) {

//                try {

//                    Thread.sleep(100);

//                } catch (InterruptedException e) {

//                    e.printStackTrace();

//                }

//                System.out.println(Thread.currentThread().getName() + "正在出售第" + tickets + "張票");

//                tickets--;

//            }

//        }

//    }

//    private synchronized void sellTicket(){

//            if (tickets > 0) {

//                try {

//                    Thread.sleep(100);

//                } catch (InterruptedException e) {

//                    e.printStackTrace();

//                }

//                System.out.println(Thread.currentThread().getName() + "正在出售第" + tickets + "張票");

//                tickets--;

//            }

    private static synchronized void sellTicket(){

        if (tickets > 0) {

            try {

                Thread.sleep(100);

            } catch (InterruptedException e) {

                e.printStackTrace();

            }

            System.out.println(Thread.currentThread().getName() + "正在出售第" + tickets + "張票");

            tickets--;

        }

    }

    public static void main(String[] args) {

        SellTicket sellTicket = new SellTicket();

        Thread thread1 = new Thread(sellTicket, "視窗1");

        Thread thread2 = new Thread(sellTicket, "視窗2");

        Thread thread3 = new Thread(sellTicket, "視窗3");

        thread1.start();

        thread2.start();

        thread3.start();

    }

}

lock鎖

lock實現提供比使用synchronized方法和語句可獲得更廣泛的操作

• void lock()獲得鎖
• void unlock()釋放

lock是介面不能直接例項化，採用實現類例項化ReentrantLock

import java.util.concurrent.locks.Lock;

import java.util.concurrent.locks.ReentrantLock;

public class SellTicket implements Runnable {

    private int tickets = 100;

    private Object object = new Object();

    private Lock lock = new ReentrantLock();

    @Override

    public void run() {

        while (true) {

            try {

                lock.lock();

                if (tickets > 0) {

                    try {

                        Thread.sleep(100);

                    } catch (InterruptedException e) {

                        e.printStackTrace();

                    }

                    System.out.println(Thread.currentThread().getName() + "正在出售第" + tickets + "張票");

                    tickets--;

                }

            } finally {

                lock.unlock();

            }

        }

    }

    public static void main(String[] args) {

        SellTicket sellTicket = new SellTicket();

        Thread thread1 = new Thread(sellTicket, "視窗1");

        Thread thread2 = new Thread(sellTicket, "視窗2");

        Thread thread3 = new Thread(sellTicket, "視窗3");

        thread1.start();

        thread2.start();

        thread3.start();

    }

}