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Java并发编程实战（使用synchronized实现同步方法）

本文介绍java最基本的同步方式，即使用synchronized关键字来控制一个方法的并发访问，如果一个对象已用synchronized关键字声明，那么只有一个执行线程允许去访问它，其它试图访问这个对象的线程将被挂起，直到第一个线程访问完毕。

下面通过一个小例子来学习这个概念，公司向银行存钱，取钱场景。

1：创建Account的账号类，它是银行账户的模型，只有一个双精度浮点型属性，balance.

2：实现balance的get set 方法。

3：实现AddAmount()方法，将传入的数量加到余额balance中，并且在同一时间只允许一个线程去改变这个值，使用synchronized关键字。

4：实现SubtractAmount()方法，将传入的数量从余额balance中扣除，并且在同一时间只允许一个线程去改变这个值。

具体代码：

 1 public class Account {  2   3     /**  4      * Balance of the bank account  5      */  6     private double balance;  7   8     /**  9      * Returns the balance of the account 10      * @return the balance of the account 11      */ 12     public double getBalance() { 13         return balance; 14     } 15  16     /** 17      * Establish the balance of the account 18      * @param balance the new balance of the account 19      */ 20     public void setBalance(double balance) { 21         this.balance = balance; 22     } 23      24     /** 25      * Add an import to the balance of the account 26      * @param amount import to add to the balance 27      */ 28     public synchronized void addAmount(double amount) { 29             double tmp=balance; 30             try { 31                 Thread.sleep(10); 32             } catch (InterruptedException e) { 33                 e.printStackTrace(); 34             } 35             tmp+=amount; 36     } 37      38     /** 39      * Subtract an import to the balance of the account 40      * @param amount import to subtract to the balance 41      */ 42     public synchronized void subtractAmount(double amount) { 43             double tmp=balance; 44             try { 45                 Thread.sleep(10); 46             } catch (InterruptedException e) { 47                 e.printStackTrace(); 48             } 49             tmp-=amount; 50             balance=tmp; 51     } 52      53 }

5：实现一个ATM模拟类Bank，它使用subtractAmount()方法对账户的余额进行扣除，实现Runabl接口。

具体代码：

 1 public class Bank implements Runnable {  2   3     /**  4      * The account affected by the operations  5      */  6     private Account account;  7       8     /**  9      * Constructor of the class. Initializes the account 10      * @param account The account affected by the operations 11      */ 12     public Bank(Account account) { 13         this.account=account; 14     } 15      16      17     /** 18      * Core method of the Runnable 19      */ 20     public void run() { 21         for (int i=0; i<100; i++){ 22             account.subtractAmount(1000); 23         } 24     } 25  26 }

6：实现公司模拟类，调用addAmount()方法进行存钱，实现Runabl接口。

具体代码：

 1 public class Company implements Runnable {  2   3     /**  4      * The account affected by the operations  5      */  6     private Account account;  7       8     /**  9      * Constructor of the class. Initializes the account 10      * @param account the account affected by the operations 11      */ 12     public Company(Account account) { 13         this.account=account; 14     } 15      16     /** 17      * Core method of the Runnable 18      */ 19     public void run() { 20         for (int i=0; i<100; i++){ 21             account.addAmount(1000); 22         } 23     }

7：在主方法中调用测试：通过线程的join方法，在存期那，取钱线程模拟完毕后打印出结构。

 1 public class Main {  2   3     /**  4      * Main method of the example  5      * @param args  6      */  7     public static void main(String[] args) {  8         // Creates a new account ...  9         Account    account=new Account(); 10         // an initialize its balance to 1000 11         account.setBalance(1000); 12          13         // Creates a new Company and a Thread to run its task 14         Company    company=new Company(account); 15         Thread companyThread=new Thread(company); 16         // Creates a new Bank and a Thread to run its task 17         Bank bank=new Bank(account); 18         Thread bankThread=new Thread(bank); 19          20         // Prints the initial balance 21         System.out.printf("Account : Initial Balance: %f/n",account.getBalance()); 22          23         // Starts the Threads 24         companyThread.start(); 25         bankThread.start(); 26  27         try { 28             // Wait for the finalization of the Threads 29             companyThread.join(); 30             bankThread.join(); 31             // Print the final balance 32             System.out.printf("Account : Final Balance: %f/n",account.getBalance()); 33         } catch (InterruptedException e) { 34             e.printStackTrace(); 35         } 36     } 37 }