一、创建多线程
第二种方式：实现 Runnable 接口
步骤：
1，定义类实现Runnable接口；
2，覆盖Runnable接口中的 run 方法；
目的：将线程要运行的代码存放在该run方法中。
3，通过Thread 类建立线程对象；
4，将Runnable 接口的子类对象作为实际参数传递给Thread类的构造函数
5，调用Thread类的start 方法开启线程并调用Runnable接口子类的run方法。
二、创建多线程两种方式（继承Thread 和实现Runnable）的区别：
两种方式的区别：（线程代码存放位置不同）
继承Thread类：线程代码存放在Thread子类run方法中；
实现Runnable：线程代码存放在接口的子类的run方法中；
Java只支持单继承（即一个类只能有一个父类），但支持多实现。如果一个类本身已经继承了一个父类，如果想通过继承Thread类来启动多线程，将不可行。因此此刻已经超出继承父类的个数了。此时可以通过实现Runnable接口，来启动多线程。
实现方式的优点：避免单继承的局限性
在定义线程的时候，推荐使用实现的方式创建线程
public class TicketThread {
public static void main(String[] args) {
// TODO Auto-generated method stub
Ticket t = new Ticket();
Thread t1 = new Thread(t);
Thread t2 = new Thread(t);
Thread t3 = new Thread(t);
t1.start();
t2.start();
t3.start();
}
}
class Ticket implements Runnable{
private static int ticket = 50;
public void run() {
while (true) {
if (ticket==0) {
break;
}
if (ticket>0) {
System.out.println(Thread.currentThread().getName()+ " sale: "+ ticket--);
}
}
}
}执行结果：
Thread-0 sale: 50
Thread-1 sale: 48
Thread-2 sale: 49
Thread-1 sale: 46
Thread-1 sale: 44
Thread-0 sale: 47
Thread-0 sale: 42
Thread-0 sale: 41
Thread-0 sale: 40
Thread-0 sale: 39
Thread-0 sale: 38
Thread-0 sale: 37
Thread-0 sale: 36
Thread-0 sale: 35
Thread-0 sale: 34
Thread-0 sale: 33
Thread-0 sale: 32
Thread-0 sale: 31
Thread-0 sale: 30
Thread-0 sale: 29
Thread-0 sale: 28
Thread-1 sale: 43
Thread-2 sale: 45
Thread-2 sale: 25
Thread-2 sale: 24
Thread-2 sale: 23
Thread-2 sale: 22
Thread-2 sale: 21
Thread-1 sale: 26
Thread-0 sale: 27
Thread-0 sale: 18
Thread-0 sale: 17
Thread-0 sale: 16
Thread-0 sale: 15
Thread-1 sale: 19
Thread-2 sale: 20
Thread-2 sale: 12
Thread-2 sale: 11
Thread-2 sale: 10
Thread-2 sale: 9
Thread-2 sale: 8
Thread-2 sale: 7
Thread-2 sale: 6
Thread-2 sale: 5
Thread-2 sale: 4
Thread-2 sale: 3
Thread-2 sale: 2
Thread-1 sale: 13
Thread-0 sale: 14
Thread-2 sale: 1
三、多线程同步代码块
问题描述：50张票，每执行一次run方法，票数减一，直至ticket 为0；代码描述情景：
public class TicketThread {
public static void main(String[] args) {
// TODO Auto-generated method stub
Ticket t = new Ticket();
Thread t1 = new Thread(t);
Thread t2 = new Thread(t);
Thread t3 = new Thread(t);
t1.start();
t2.start();
t3.start();
}
}
class Ticket implements Runnable{
private static int ticket = 50;
public void run() {
while (true) {
if (ticket>0) {
//此处用异常模拟系统切换到其他线程中执行操作
try {Thread.sleep(10);} catch (Exception e) {}
System.out.println(Thread.currentThread().getName()+ " sale: "+ ticket--);
}
}
}
}执行结果如下：
Thread-2 sale: 50
Thread-1 sale: 49
Thread-0 sale: 48
Thread-1 sale: 47
Thread-2 sale: 46
Thread-2 sale: 45
Thread-1 sale: 44
Thread-0 sale: 43
Thread-1 sale: 42
Thread-2 sale: 41
Thread-1 sale: 40
Thread-2 sale: 39
Thread-0 sale: 38
Thread-2 sale: 37
Thread-1 sale: 36
Thread-0 sale: 35
Thread-1 sale: 34
Thread-2 sale: 33
Thread-1 sale: 32
Thread-2 sale: 31
Thread-0 sale: 30
Thread-2 sale: 29
Thread-1 sale: 28
Thread-0 sale: 27
Thread-1 sale: 26
Thread-2 sale: 25
Thread-0 sale: 24
Thread-2 sale: 23
Thread-1 sale: 22
Thread-1 sale: 21
Thread-2 sale: 20
Thread-1 sale: 19
Thread-2 sale: 18
Thread-0 sale: 17
Thread-2 sale: 16
Thread-1 sale: 15
Thread-1 sale: 14
Thread-2 sale: 13
Thread-0 sale: 12
Thread-2 sale: 11
Thread-1 sale: 10
Thread-0 sale: 9
Thread-1 sale: 8
Thread-2 sale: 7
Thread-2 sale: 6
Thread-1 sale: 5
Thread-0 sale: 4
Thread-1 sale: 3
Thread-2 sale: 2
Thread-0 sale: 1
Thread-1 sale: 0
Thread-2 sale: -1
出现了ticket为负数的情况，但是代码中循环只有ticket大于0才执行减一的操作，因此线程出现不安全的情况。
问题原因：
当多条语句操作同一个线程的共享数据时，一个线程对多条语句只执行了一部分操作，而且还没执行完，另一个线程参与进
来，导致共享数据的错误；
同步代码块的格式：
synchronized(对象){
// 需要被同步的代码块
}
场景类似：排队使用独立卫生间
同步的前提：
1，必须要有两个或者两个以上的线程。
2，必须是多个线程使用同一个锁
对象如同锁。持有锁的线程可以在同步中执行。没有持有锁的线程即使获取cpu的执行权，也进不去，因为没有获取锁。
此时可以通过加入同步代码块解决问题，代码如下：
package Thread;
public class TicketThread {
public static void main(String[] args) {
// TODO Auto-generated method stub
Ticket t = new Ticket();
Thread t1 = new Thread(t);
Thread t2 = new Thread(t);
Thread t3 = new Thread(t);
t1.start();
t2.start();
t3.start();
}
}
class Ticket implements Runnable{
private static int ticket = 50;
Object obj = new Object();
public void run() {
while (true) {
synchronized (obj) {
if (ticket>0) {
try {Thread.sleep(10);} catch (Exception e) {}
System.out.println(Thread.currentThread().getName()+ " sale: "+ ticket--);
}
}
}
}
}执行结果
Thread-2 sale: 50
Thread-2 sale: 49
Thread-2 sale: 48
Thread-2 sale: 47
Thread-2 sale: 46
Thread-2 sale: 45
Thread-2 sale: 44
Thread-2 sale: 43
Thread-2 sale: 42
Thread-2 sale: 41
Thread-2 sale: 40
Thread-2 sale: 39
Thread-2 sale: 38
Thread-2 sale: 37
Thread-2 sale: 36
Thread-2 sale: 35
Thread-2 sale: 34
Thread-2 sale: 33
Thread-2 sale: 32
Thread-2 sale: 31
Thread-2 sale: 30
Thread-2 sale: 29
Thread-2 sale: 28
Thread-2 sale: 27
Thread-2 sale: 26
Thread-2 sale: 25
Thread-2 sale: 24
Thread-2 sale: 23
Thread-2 sale: 22
Thread-2 sale: 21
Thread-2 sale: 20
Thread-2 sale: 19
Thread-2 sale: 18
Thread-2 sale: 17
Thread-2 sale: 16
Thread-2 sale: 15
Thread-2 sale: 14
Thread-2 sale: 13
Thread-2 sale: 12
Thread-2 sale: 11
Thread-2 sale: 10
Thread-2 sale: 9
Thread-2 sale: 8
Thread-2 sale: 7
Thread-2 sale: 6
Thread-2 sale: 5
Thread-2 sale: 4
Thread-2 sale: 3
Thread-2 sale: 2
Thread-2 sale: 1
同步代码块优缺点
优点：解决多线程安全问题；
缺点：多个线程需要判断锁，较为消耗资源；
多线程找安全问题的方法：
1，明确哪些代码是多线程运行代码；
2，明确共享数据；
3，明确多线程运行代码中哪些语句是操作共享数据的；
四、同步函数
将 synchronized 以关键字的形式放在函数定义上，该函数就是同步函数，此时锁是“this”
例如：
public class TicketThread {
public static void main(String[] args) {
// TODO Auto-generated method stub
Ticket t = new Ticket();
Thread t1 = new Thread(t);
Thread t2 = new Thread(t);
Thread t3 = new Thread(t);
t1.start();
t2.start();
t3.start();
}
}
class Ticket implements Runnable{
private static int ticket = 1000;
public void run() {
while (true) {
show();
}
}
//同步函数
public synchronized void show() {
if (ticket>0) {
try {Thread.sleep(10);} catch (Exception e) {}
System.out.println(Thread.currentThread().getName()+ " sale: "+ ticket--);
}
}
}静态的同步方法，使用的锁是该方法所在类的字节码文件对象。 类名.class 该对象的类型是class
//懒汉式的作用：延迟加载；
class Single {
private static Single s = null;
private Single() {}
public static Single getInstance()
{
//用双重判断的形式，解决同步锁的低效问题
if (s==null)
{
synchronized (Single.class)
{
if (s == null)
{
s = new Single();
}
}
}
return s;
}
}