Java StringBuffer在线程安全程序的设计中是如何工作的？

许多人提到了Java中StringBuffer和StringBuilder之间的区别。StringBuffer包含同步方法。人们会说“如果一个缓冲区被许多线程使用，那么就使用StringBuffer。”但是使用StringBuffer真的能保证“线程安全”吗？

好吧，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");
        }

    }
}

当然，它是“线程安全的”，即不存在数据竞争（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，使其产生合理的结果。StringBuffer附带的锁没有任何帮助。”

---

我希望这对您有深刻的见解！

好吧，我认为强调StringBuffer的一些实际用途是很重要的。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");
        }

    }
}

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

---

我希望这对您有深刻的见解！

好吧，我认为强调StringBuffer的一些实际用途是很重要的。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");
        }

    }
}

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

---

我希望这对您有深刻的见解！

好吧，我认为强调StringBuffer的一些实际用途是很重要的。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");
        }

    }
}

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

---

---

我希望这对你有深刻的见解！

提问和回答。这是怎么回事？？？@Youngistan是的，这是允许的。@Youngistan您好..stackoverflow给了我一个分享知识和问答风格的选择。Ohkk我不知道这一点。我已经将此标记为版主注意。问这个问题只是为了在博客上展示答案。它更适合到一篇博客文章。它还复制了其他线程。这个答案可以发布到其他线程中，不会丢失任何东西。询问和回答。这是什么？？@Youngistan是的，这是允许的。@Youngistan Hello..stackoverflow给了我一个分享知识、问答风格的选项。Ohkk我没有意识到这一点。我已将此标记为版主att注意。问这个问题只是为了在博客上展示答案。它更适合博客帖子。它还复制了其他线程。这个答案可以发布到其他线程中，不会丢失任何内容。问和答。这是什么？？@Youngistan是的，这是允许的。@Youngistan Hello..stackoverflow给了我一个选择sha的选项re knowledge，Q&A风格。Ohkk我没有意识到这一点。我已经将此标记为版主注意。问这个问题只是为了在博客上展示答案。它更适合于博客帖子。它还复制了其他帖子。这个答案可以发布到其他帖子中，不会丢失任何内容。提问和回答。这是什么@Youngistan是的，这是允许的。@Youngistan Hello..stackoverflow为我提供了一个分享知识和问答风格的选项。哦，我不知道这一点。我已将此标记为版主注意。问这个问题只是为了在博客上展示答案。它更适合博客文章。它还复制了其他线程。这个答案可能是博文将其绑定到其他线程之一，不会丢失任何内容。