线程同步是指对共享资源，若不是共享资源则不涉及线程同步问题。

synchronized方法可以锁定对象，也可以锁定代码块。

当锁定对象时，修饰在类的方法前面。记住：若两个线程同时访问一个对象的不同方法，且对象有一个synchronized方法和一个非synchronized方法，则是两个线程并不涉及同步问题。只有当访问同一个对象的synchronsized方法（可以是不同的synchronized方法）时才会有同步。

比如对象toy有如下两个方法

package concurrent1;public class toy {private int number;public toy(){number = 100;}synchronized public void paint(){System.out.println("thread:"+Thread.currentThread().getName() + ":paint begin");try{int i = (int) (Math.random()*1000);Thread.sleep(i);}catch(Exception e){}System.out.println("thread:"+Thread.currentThread().getName()+":paint");System.out.println("thread:"+Thread.currentThread().getName() + ":paint end");}synchronized public void write(){System.out.println("thread:"+Thread.currentThread().getName() + ":write begin");try{int i = (int) (Math.random()*1000);Thread.sleep(i);}catch(Exception e){}System.out.println("thread:"+Thread.currentThread().getName()+":write");System.out.println("thread:"+Thread.currentThread().getName() + ":write end");}
}

package concurrent1;public class Test {public static void main(String[] args) {// TODO Auto-generated method stubtoy mytoy = new toy();boy myboy = new boy(mytoy);boy youboy = new boy(mytoy);myboy.setName("myboy");youboy.setName("youboy");myboy.start();youboy.start();}}

结果：

thread:myboy:write begin
thread:myboy:write
thread:myboy:write end
thread:myboy:paint begin
thread:myboy:paint
thread:myboy:paint end
thread:youboy:write begin
thread:youboy:write
thread:youboy:write end
thread:youboy:paint begin
thread:youboy:paint
thread:youboy:paint end

若将paint方法前的synchronized取消掉，输出如下，可以看出write方法是异步执行的，只有paint方法是同步执行的。

thread:myboy:write begin
thread:youboy:write begin
thread:myboy:write
thread:myboy:write end
thread:myboy:paint begin
thread:myboy:paint
thread:myboy:paint end
thread:youboy:write
thread:youboy:write end
thread:youboy:paint begin
thread:youboy:paint
thread:youboy:paint end