synchronized加鎖String踩坑日記

synchronized加鎖String踩坑日記

本文參考 juejin.im/post/59fffd… JVM內存分佈

咱們知道String.intern()方法會判斷該字符串是否存在常量池中,若是存在直接獲取,不存在將當前字符串放到常量池中

1. String 分狀況存在於常量池跟JVM堆中,首先讓咱們來看一下例子一:

public class TestStringSycn {
    public static void main(String[] args) {
        String a = "a"+"b";
        System.out.println(a);
    }
}
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咱們使用javap -c TestStringSycn.class來看編譯器如何幫咱們解析的:

$ javap -c TestStringSycn.class
Compiled from "TestStringSycn.java"
public class TestStringSycn {
  public TestStringSycn();
    Code:
       0: aload_0
       1: invokespecial #1                  // Method java/lang/Object."<init>":()V
       4: return

  public static void main(java.lang.String[]);
    Code:
       0: ldc           #2                  // String ab
       2: astore_1
       3: getstatic     #3                  // Field java/lang/System.out:Ljava/io/PrintStream;
       6: aload_1
       7: invokevirtual #4                  // Method java/io/PrintStream.println:(Ljava/lang/String;)V
      10: return
}
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咱們能夠看到,編輯器直接是幫咱們直接拼接在一塊兒，也就是說這裏其實會在常量池裏面存在 ab 這個常量 2. 例子二：

public class TestStringSycn {
    public static void main(String[] args) {
        String a = "key";
        String b = "zzz";
        String c = a+ b;
        System.out.println(c);
    }
}
$ javap -c TestStringSycn.class
Compiled from "TestStringSycn.java"
public TestStringSycn {
  public TestStringSycn();
    Code:
       0: aload_0
       1: invokespecial #1                  // Method java/lang/Object."<init>":()V
       4: return

  public static void main(java.lang.String[]);
    Code:
       0: ldc           #2                  // String key
       2: astore_1
       3: ldc           #3                  // String zzz
       5: astore_2
       6: new           #4                  // class java/lang/StringBuilder
       9: dup
      10: invokespecial #5                  // Method java/lang/StringBuilder."<init>":()V
      13: aload_1
      14: invokevirtual #6                  // Method java/lang/StringBuilder.append:(Ljava/lang/String;)Ljava/lang/StringBuilder;
      17: aload_2
      18: invokevirtual #6                  // Method java/lang/StringBuilder.append:(Ljava/lang/String;)Ljava/lang/StringBuilder;
      21: invokevirtual #7                  // Method java/lang/StringBuilder.toString:()Ljava/lang/String;
      24: astore_3
      25: getstatic     #8                  // Field java/lang/System.out:Ljava/io/PrintStream;
      28: aload_3
      29: invokevirtual #9                  // Method java/io/PrintStream.println:(Ljava/lang/String;)V
      32: return
}
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咱們能夠看到,這裏採用的是StringBuilder直接幫咱們拼接而成，最後調用toString()方法,因此每次都會在堆上面生成一個String對象。

回到主題

當咱們使用synchronized加鎖String的時候,咱們須要保證當前加鎖的key是惟一的,經過例子1，2知道。若是要加鎖String,最好是加鎖String.intern()方法。eg:

public class TestStringSycn {
    public static void main(String[] args) {
        String type = "order";
        String id = "1";
        String key = type + id;
        synchronized (key.intern()) {
            //執行方法
        }
    }
}
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