How does StringBuffer really work in the design of thread-safe programs?

Question

Many people have mentioned that the difference between StringBuffer and StringBuilder in Java. StringBuffer contains synchronized methods. And people say things like "if a buffer is used by many threads, use StringBuffer." But is the use of StringBuffer really going to guarantee "thread-safety"?

Answer 1

Well, I think it is important to highlight some actual uses of StringBuffer. To that end, I have designed a simple program to illustrate how StringBuffer can be superior to StringBuilder in achieving thread-safety.

/**
 * 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");
        }

    }
}

However, like many others have pointed out also, StringBuffer is not a miracle cure for designing thread-safe applications. To go even further, I would say that the tools and libraries for concurrency (e.g. Vector) should be well-understood and appropriately implemented, and we should not make easy assumptions about using "thread-safe" libraries.

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

Jeremy's example illustrates the point, and I quote:

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

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

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

"Sure, it is "thread-safe", in the sense that there are no data races (which are basically concurrent accesses without adequate synchronization). But you have no idea what thread 3 will print: "ab" or "ba". I would have to introduce more synchronization to make this produce a sensible result. The locks that come with the StringBuffer haven't helped anything."

I hope this is insightful for you!