我认为强调StringBuffer的实际用途很重要。为此，我设计了一个简单的程序，以说明StringBuffer如何在实现线程安全方面比StringBuilder更为优越。

/**
 * 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，正如其他许多人所指出的，StringBuffer并不是设计线程安全应用程序的灵丹妙药。更进一步地说，我认为并发工具和库（例如Vector）应该得到充分理解和适当实现，我们不应该轻易地假设使用“线程安全”的库。

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

Jeremy的例子说明了这一点，我引用他的话：

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）。但你不知道线程3将打印什么：“ab”还是“ba” 。我必须引入更多的同步来使其产生一个合理的结果。StringBuffer带来的锁并没有起到任何作用。"

希望这对你有启示！