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：
public class Account { /**
* Balance of the bank account
*/
private double balance; /**
* Returns the balance of the account
* @return the balance of the account
*/
public double getBalance() {
return balance;
} /**
* Establish the balance of the account
* @param balance the new balance of the account
*/
public void setBalance(double balance) {
this.balance = balance;
} /**
* Add an import to the balance of the account
* @param amount import to add to the balance
*/
public synchronized void addAmount(double amount) {
double tmp=balance;
try {
Thread.sleep(10);
} catch (InterruptedException e) {
e.printStackTrace();
}
tmp+=amount;
balance=tmp;
} /**
* Subtract an import to the balance of the account
* @param amount import to subtract to the balance
*/
public synchronized void subtractAmount(double amount) {
double tmp=balance;
try {
Thread.sleep(10);
} catch (InterruptedException e) {
e.printStackTrace();
}
tmp-=amount;
balance=tmp;
} }
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：
public class Bank implements Runnable { /**
* The account affected by the operations
*/
private Account account; /**
* Constructor of the class. Initializes the account
* @param account The account affected by the operations
*/
public Bank(Account account) {
this.account=account;
} /**
* Core method of the Runnable
*/
public void run() {
for (int i=0; i<100; i++){
account.subtractAmount(1000);
}
} }
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：
public class Company implements Runnable { /**
* The account affected by the operations
*/
private Account account; /**
* Constructor of the class. Initializes the account
* @param account the account affected by the operations
*/
public Company(Account account) {
this.account=account;
} /**
* Core method of the Runnable
*/
public void run() {
for (int i=0; i<100; i++){
account.addAmount(1000);
}
}
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。
public class Main { /**
* Main method of the example
* @param args
*/
public static void main(String[] args) {
// Creates a new account ...
Account account=new Account();
// an initialize its balance to 1000
account.setBalance(1000); // Creates a new Company and a Thread to run its task
Company company=new Company(account);
Thread companyThread=new Thread(company);
// Creates a new Bank and a Thread to run its task
Bank bank=new Bank(account);
Thread bankThread=new Thread(bank); // Prints the initial balance
System.out.printf("Account : Initial Balance: %f\n",account.getBalance()); // Starts the Threads
companyThread.start();
bankThread.start(); try {
// Wait for the finalization of the Threads
companyThread.join();
bankThread.join();
// Print the final balance
System.out.printf("Account : Final Balance: %f\n",account.getBalance());
} catch (InterruptedException e) {
e.printStackTrace();
}
}
}
结果,相同时间内，存与取执行后应该是相等的。如果我们在方法中不去使用synchronized关键字，那么得出的结果就不对了。
Account : Initial Balance: 1000.000000
Account : Final Balance: 1000.000000
