java - StringBuffer 在线程安全程序的设计中到底是如何工作的？

Question

很多人都提到过Java中StringBuffer和StringBuilder的区别。 StringBuffer 包含同步方法。人们会说“如果一个缓冲区被多个线程使用，就使用 StringBuffer”。但是使用StringBuffer真的能保证“线程安全”吗？

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 在实现线程安全方面如何优于 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");
        }

    }
}

但是，正如许多其他人也指出的那样，StringBuffer 并不是设计线程安全应用程序的 Elixir 。更进一步，我想说用于并发的工具和库(例如， Vector)应该被很好地理解并适本地实现，我们不应该对使用“线程安全”库做出简单的假设。

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

杰里米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());

“当然，它是“线程安全”的，因为没有数据竞争(基本上是没有足够同步的并发访问)。但是您不知道线程 3 将打印什么:“ab”或“ba"。我必须引入更多的同步才能产生合理的结果。StringBuffer 附带的锁没有任何帮助。"

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