StringBuilder与StringBuffer相比为什么不是线程安全的

StringBuilder线程不安全的例子

package com.xiva.demo;
public class StringBuilderDemo extends Thread { 
	private StringBuilder sb; 
	public StringBuilderDemo(String name,StringBuilder sb) { 
		super(name); 
		this.sb = sb; 
	} 
	
	@Override 
	public void run() { 
		for (int i = 0; i < 10000; i++) { 
			sb.append("abc");
//			System.out.println(Thread.currentThread().getName()+sb.toString());
		} 
	} 
 

	public static void main(String[] args) { 
		StringBuilder sBuilder = new StringBuilder(); 
		StringBuilderDemo mt = new StringBuilderDemo("sBuilder线程",sBuilder); 
//		new Thread(mt).start();
		mt.start();
		for (int i = 0; i < 10000; i++) { 
			sBuilder.append("abcd"); 
		}
		System.out.println("main"+sBuilder.length());
//		System.out.println("main"+sBuilder.toString()); 
	}
} 

 main所打印出的sBuilder的长度不定，也就是说mt线程执行的时候，main函数对sBuilder仍在影响，而下面的例子中，所打印出的（线程是安全的）长度是一定的！

 

 

 

package com.xiva.demo;

public class StringBufferDemo extends Thread { 
	private StringBuffer sBuffer; 
	public StringBufferDemo(String name,StringBuffer sb) { 
		super(name); 
		this.sBuffer = sb; 
	} 
	
	@Override 
	public void run() { 
		for (int i = 0; i < 10000; i++) { 
			sBuffer.append("abc");
//			System.out.println(Thread.currentThread().getName()+sb.toString());
		}
		
	} 
 

	public static void main(String[] args) { 
		StringBuffer sBuffer = new StringBuffer(); 
		StringBufferDemo mt = new StringBufferDemo("sBuffer线程",sBuffer);
		mt.start();
//		new Thread(mt).start();
//		new Thread(mt).start();
		for (int i = 0; i < 10000; i++) { 
			sBuffer.append("abcd"); 
		}
		while(mt.isAlive()){ 
		}
		System.out.println(sBuffer.length()); 
	}
}

评论

1 楼 QQ371496669 2012-09-20  

能否具体讲解一下为什么StringBuilder的长度会不一样。

如果在StringBuilder里把append这块用synchronized (sBuilder) 包围起来，main线程也一样

那么，执行结果就和StringBuffer一样了。

请问大虾能否解释一下我的疑问呢？