当使用synchronized关键字来保护代码块时，必须把对象引用作为传入参数。通常情况下，使用this关键字来引用执行方法所属的对象，也可以使用其他的对象对其进行引用。 一般来说，这些对象就是为这个目的而创建的。例如，在类中有两个非依赖属性，它们被 多个线程共享，你必须同步每一个变量的访问，但是同一时刻只允许一个线程访问一个属性变量，其他某个线程访问另一个属性变量。
在本节中，我们将通过范例学习实现电影院售票场景的编程。这个范例模拟了有两个屏幕和两个售票处的电影院，一个售票处卖出的一张票，只能用于其中一个电影院，不能同时用于两个电影院，因此每个电影院的剩余票数是独立的属性。
package com.packtpub.java7.concurrency.chapter2.recipe2.core;
import com.packtpub.java7.concurrency.chapter2.recipe2.task.Cinema;
import com.packtpub.java7.concurrency.chapter2.recipe2.task.TicketOffice1;
import com.packtpub.java7.concurrency.chapter2.recipe2.task.TicketOffice2;
/**
* Core class of the example. Creates a cinema and two threads for
* the ticket office. Run the threads to analyze the results obtained
*
*/
public class Main {
/**
* Main method of the example
* @param args
*/
public static void main(String[] args) {
// Creates a Cinema
Cinema cinema=new Cinema();
// Creates a TicketOffice1 and a Thread to run it
TicketOffice1 ticketOffice1=new TicketOffice1(cinema);
Thread thread1=new Thread(ticketOffice1,"TicketOffice1");
// Creates a TicketOffice2 and a Thread to run it
TicketOffice2 ticketOffice2=new TicketOffice2(cinema);
Thread thread2=new Thread(ticketOffice2,"TicketOffice2");
// Starts the threads
thread1.start();
thread2.start();
try {
// Waits for the finalization of the threads
thread1.join();
thread2.join();
} catch (InterruptedException e) {
e.printStackTrace();
}
// Print the vacancies in the cinemas
System.out.printf("Room 1 Vacancies: %d\n",cinema.getVacanciesCinema1());
System.out.printf("Room 2 Vacancies: %d\n",cinema.getVacanciesCinema2());
}
}
package com.packtpub.java7.concurrency.chapter2.recipe2.task;
public class Cinema {
/**
* This two variables store the vacancies in two cinemas
*/
private long vacanciesCinema1;
private long vacanciesCinema2;
/**
* Two objects for the synchronization. ControlCinema1 synchronizes the
* access to the vacancesCinema1 attribute and controlCinema2 synchronizes
* the access to the vacanciesCinema2 attribute.
*/
private final Object controlCinema1,controlCinema2;
/**
* Constructor of the class. Initializes the objects
*/
public Cinema(){
controlCinema1=new Object();
controlCinema2=new Object();
vacanciesCinema1=20;
vacanciesCinema2=20;
}
/**
* This method implements the operation of sell tickets for the cinema 1
* @param number number of tickets sold
* @return true if the tickets are sold,false if there is no vacancies
*/
public boolean sellTickets1 (int number) {
synchronized (controlCinema1) {
if (number<vacanciesCinema1) {
vacanciesCinema1-=number;
return true;
} else {
return false;
}
}
}
/**
* This method implements the operation of sell tickets for the cinema 2
* @param number number of tickets sold
* @return true if the tickets are sold,false if there is no vacancies
*/
public boolean sellTickets2 (int number){
synchronized (controlCinema2) {
if (number<vacanciesCinema2) {
vacanciesCinema2-=number;
return true;
} else {
return false;
}
}
}
/**
* This method implements the operation of return tickets for the cinema 1
* @param number number of the tickets returned
* @return true
*/
public boolean returnTickets1 (int number) {
synchronized (controlCinema1) {
vacanciesCinema1+=number;
return true;
}
}
/**
* This method implements the operation of return tickets for the cinema 1
* @param number number of the tickets returned
* @return true
*/
public boolean returnTickets2 (int number) {
synchronized (controlCinema2) {
vacanciesCinema2+=number;
return true;
}
}
/**
* Return the vacancies in the cinema 1
* @return the vacancies in the cinema 1
*/
public long getVacanciesCinema1() {
return vacanciesCinema1;
}
/**
* Return the vacancies in the cinema 2
* @return the vacancies in the cinema 2
*/
public long getVacanciesCinema2() {
return vacanciesCinema2;
}
}
package com.packtpub.java7.concurrency.chapter2.recipe2.task;
/**
* This class simulates a ticket office. It sell or return tickets
* for the two cinemas
*
*/
public class TicketOffice1 implements Runnable {
/**
* The cinema
*/
private Cinema cinema;
/**
* Constructor of the class
* @param cinema the cinema
*/
public TicketOffice1 (Cinema cinema) {
this.cinema=cinema;
}
/**
* Core method of this ticket office. Simulates selling and returning tickets
*/
@Override
public void run() {
cinema.sellTickets1(3);
cinema.sellTickets1(2);
cinema.sellTickets2(2);
cinema.returnTickets1(3);
cinema.sellTickets1(5);
cinema.sellTickets2(2);
cinema.sellTickets2(2);
cinema.sellTickets2(2);
}
}
package com.packtpub.java7.concurrency.chapter2.recipe2.task;
/**
* This class simulates a ticket office. It sell or return tickets
* for the two cinemas
*
*/
public class TicketOffice2 implements Runnable {
/**
* The cinema
*/
private Cinema cinema;
/**
* Constructor of the class
* @param cinema the cinema
*/
public TicketOffice2(Cinema cinema){
this.cinema=cinema;
}
/**
* Core method of this ticket office. Simulates selling and returning tickets
*/
@Override
public void run() {
cinema.sellTickets2(2);
cinema.sellTickets2(4);
cinema.sellTickets1(2);
cinema.sellTickets1(1);
cinema.returnTickets2(2);
cinema.sellTickets1(3);
cinema.sellTickets2(2);
cinema.sellTickets1(2);
}
}
用synchronized关键字保护代码块时，我们使用对象作为它的传入参数。JVM保证同一时间只有一个线程能够访问这个对象的代码保护块（注意我们一直谈论的是对象不是类)。
备注：这个例子使用了一个对象来控制对vacanciesCinema1属性的访问，所以同一时间只有一个线程能够修改这个属性：使用了另一个对象来控制vacanciesCinema2属性的访问，所以同一时间只有一个线程能够修改这个属性。但是，这个例子允许同时运行两个线 程：一个修改vacancesCinemal属性，另一个修改vacanciesCinema2属性。
运行这个范例，可以看到最终结果总是与每个电影院的剩余票数一致。
转自：百度快照
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