工作中涉及到了一個普通的http服務(wù),接收http請求,業(yè)務(wù)中需要保證同一個user只能同時有一個線程在處理,需要手動加鎖.
由于只部署單個實例,所以沒有使用分布式鎖,第一想到的還是一個synchronized,使用synchronized對user加鎖.測試代碼如下:

/**
 * @author: hman
 * @date 2019-04-25
 * @desc: //TODO
 */
public class TestSynchronized {


  public static void main(String[] args) {
    ExecutorService executorService = Executors.newFixedThreadPool(10);
    Random random = new Random();
    //10個線程
    for (int i = 0; i < 10; i++) {
      executorService.execute(() -> {
        //使用隨機數(shù)模擬user并發(fā)
        test(random.nextInt(3));
      });

    }
  }

  private static void test(int a) {
//隨機字符串+數(shù)字
    String s = "asdfasdf";
    String lock = s + a;
    synchronized (lock) {
      try {
        Thread.sleep(2000);
      } catch (InterruptedException e) {
        e.printStackTrace();
      }
      System.out.println(lock);
    }
  }
}

打印出來的結(jié)果

asdfasdf0
asdfasdf0
asdfasdf2
asdfasdf1
asdfasdf1
asdfasdf0
asdfasdf0
asdfasdf2
asdfasdf2
asdfasdf0

明顯是沒鎖到.

在網(wǎng)上查了查原因,發(fā)現(xiàn)synchronized比較鎖是使用的==,比較的是內(nèi)存地址,因為每次都會new一個String,所以內(nèi)存肯定不同.

解決思路:
1.改成小于-127->127的Integer,因為在這個范圍的Integer是直接從常量池中取得.建議直接使用key取hash取模128即可
2.也是最簡單最有效的方法.使用String的intern方法.

/**
   * Returns a canonical representation for the string object.
   * <p>
   * A pool of strings, initially empty, is maintained privately by the
   * class {@code String}.
   * <p>
   * When the intern method is invoked, if the pool already contains a
   * string equal to this {@code String} object as determined by
   * the {@link #equals(Object)} method, then the string from the pool is
   * returned. Otherwise, this {@code String} object is added to the
   * pool and a reference to this {@code String} object is returned.
   * <p>
   * It follows that for any two strings {@code s} and {@code t},
   * {@code s.intern() == t.intern()} is {@code true}
   * if and only if {@code s.equals(t)} is {@code true}.
   * <p>
   * All literal strings and string-valued constant expressions are
   * interned. String literals are defined in section 3.10.5 of the
   * <cite>The Java&trade; Language Specification</cite>.
   *
   * @return  a string that has the same contents as this string, but is
   *          guaranteed to be from a pool of unique strings.
   */
  public native String intern();

簡單介紹,這個intern方法在newString的時候會先去字符串常量池中判斷有沒有,如果有就用,沒有的話就new然后放進去,這樣可以完美解決這個問題

代碼修改成:

String lock = s + a;
    synchronized (lock.intern()) {
      try {
        Thread.sleep(2000);
      } catch (InterruptedException e) {
        e.printStackTrace();
      }

解決