Java良好的支持多线程。使用java,我们可以很轻松的编程一个多线程程序。但是使用多线程可能会引起并发访问的问题。synchronized和ThreadLocal都是用来解决多线程并发访问的问题。大家可能对synchronized较为熟悉，而对ThreadLocal就要陌生得多了。

    并发问题。当一个对象被两个线程同时访问时，可能有一个线程会得到不可预期的结果。

    一个简单的java类Studnet

public class Student ...{ 
private int age=0; 

public int getAge() ...{ 
return this.age; 
} 

public void setAge(int age) ...{ 
this.age = age; 
} 
} 

    一个多线程类ThreadDemo.
    这个类有一个Student的私有变量,在run方法中，它随机产生一个整数。然后设置到student变量中,从student中读取设置后的值。然后睡眠5秒钟，最后再次读student的age值。

public class ThreadDemo implements Runnable...{ 
Student student = new Student(); 
public static void main(String[] agrs) ...{ 
ThreadDemo td = new ThreadDemo(); 
Thread t1 = new Thread(td,"a"); 
Thread t2 = new Thread(td,"b"); 
t1.start(); 
t2.start(); 

} 
/**//* (non-Javadoc) 
* @see java.lang.Runnable#run() 
*/ 
public void run() ...{ 
accessStudent(); 
} 

public void accessStudent() ...{ 
String currentThreadName = Thread.currentThread().getName(); 
System.out.println(currentThreadName+" is running!"); 
// System.out.println("first read age is:"+this.student.getAge()); 
Random random = new Random(); 
int age = random.nextInt(100); 
System.out.println("thread "+currentThreadName +" set age to:"+age); 

this.student.setAge(age); 
System.out.println("thread "+currentThreadName+" first read age is:"+this.student.getAge()); 
try ...{ 
Thread.sleep(5000); 
} 
catch(InterruptedException ex) ...{ 
ex.printStackTrace(); 
} 
System.out.println("thread "+currentThreadName +" second read age is:"+this.student.getAge()); 

} 
} 

    运行这个程序,屏幕输出如下:
    a is running!
    b is running!
    thread b set age to:33
    thread b first read age is:33
    thread a set age to:81
    thread a first read age is:81
    thread b second read age is:81
    thread a second read age is:81

    需要注意的是，线程a在同一个方法中，第一次读取student的age值与第二次读取值不一致。这就是出现了并发问题。