Question

很多人都提到了Java中StringBuffer和StringBuilder的区别。 StringBuffer包含同步方法。人们会说＆＃34;如果许多线程使用缓冲区，请使用StringBuffer。＆＃34;但是StringBuffer的使用是否真的能保证＆＃34;线程安全＆＃34;？

Answer 1

嗯，我认为强调StringBuffer的一些实际用途很重要。为此，我设计了一个简单的程序来说明StringBuffer如何在实现线程安全方面优于StringBuilder。

/**
 * Run this program a couple of times. We see that the StringBuilder does not
 * give us reliable results because its methods are not thread-safe as compared
 * to StringBuffer.
 * 
 * For example, the single append in StringBuffer is thread-safe, i.e.
 * only one thread can call append() at any time and would finish writing
 * back to memory one at a time. In contrast, the append() in the StringBuilder 
 * class can be called concurrently by many threads, so the final size of the 
 * StringBuilder is sometimes less than expected.
 * 
 */
public class StringBufferVSStringBuilder {

    public static void main(String[] args) throws InterruptedException {

        int n = 10; 

        //*************************String Builder Test*******************************//
        StringBuilder sb = new StringBuilder();
        StringBuilderTest[] builderThreads = new StringBuilderTest[n];
        for (int i = 0; i < n; i++) {
            builderThreads[i] = new StringBuilderTest(sb);
        }
        for (int i = 0; i < n; i++) {
            builderThreads[i].start();
        }
        for (int i = 0; i < n; i++) {
            builderThreads[i].join();
        }
        System.out.println("StringBuilderTest: Expected result is 1000; got " + sb.length());

        //*************************String Buffer Test*******************************//

        StringBuffer sb2 = new StringBuffer();
        StringBufferTest[] bufferThreads = new StringBufferTest[n];
        for (int i = 0; i < n; i++) {
            bufferThreads[i] = new StringBufferTest(sb2);
        }
        for (int i = 0; i < n; i++) {
            bufferThreads[i].start();
        }
        for (int i = 0; i < n; i++) {
            bufferThreads[i].join();
        }
        System.out.println("StringBufferTest: Expected result is 1000; got " + sb2.length());

    }

}

// Every run would attempt to append 100 "A"s to the StringBuilder.
class StringBuilderTest extends Thread {

    StringBuilder sb;

    public StringBuilderTest (StringBuilder sb) {
        this.sb = sb;
    }

    @Override
    public void run() {
        for (int i = 0; i < 100; i++) {
            sb.append("A");
        }

    }
}


//Every run would attempt to append 100 "A"s to the StringBuffer.
class StringBufferTest extends Thread {

    StringBuffer sb2;

    public StringBufferTest (StringBuffer sb2) {
        this.sb2 = sb2;
    }

    @Override
    public void run() {
        for (int i = 0; i < 100; i++) {
            sb2.append("A");
        }

    }
}

然而，正如许多其他人所指出的那样， StringBuffer并不是设计线程安全应用程序的奇迹。为了更进一步，我会说用于并发的工具和库（例如Vector应该被很好地理解和适当地实现，我们不应该轻易地假设使用＆＃34;线程安全＆＃34;库。

http://jeremymanson.blogspot.sg/2008/08/dont-use-stringbuffer.html

Jeremy的例子说明了这一点，我引用：

Thread 1:
sb.append("a");

Thread 2:
sb.append("b");

Thread 3:
join 1,2
print(sb.toString());

＆＃34;当然，它是＆＃34;线程安全＆＃34;，在某种意义上说没有数据竞争（基本上是没有充分同步的并发访问）。但是你不知道什么线程3会打印出来：＆＃34; ab＆＃34;或＆＃34; ba＆＃34;。我将不得不引入更多的同步，以使这产生一个明智的结果。 StringBuffer附带的锁具有帮助。＆＃34;

我希望这对你很有见地！

StringBuffer如何在线程安全程序的设计中真正起作用？

1 个答案: