本文参考 juejin.im/post/59fffd… JVM内存分布java
咱们知道String.intern()方法会判断该字符串是否存在常量池中,若是存在直接获取,不存在将当前字符串放到常量池中app
public class TestStringSycn {
public static void main(String[] args) {
String a = "a"+"b";
System.out.println(a);
}
}
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咱们使用javap -c TestStringSycn.class
来看编译器如何帮咱们解析的:编辑器
$ 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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咱们能够看到,编辑器直接是帮咱们直接拼接在一块儿,也就是说这里其实会在常量池里面存在 ab 这个常量 2. 例子二:post
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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咱们能够看到,这里采用的是StringBuilder直接帮咱们拼接而成,最后调用toString()方法,因此每次都会在堆上面生成一个String对象。ui
当咱们使用synchronized
加锁String的时候,咱们须要保证当前加锁的key是惟一的,经过例子1,2知道。若是要加锁String,最好是加锁String.intern()方法。eg:spa
public class TestStringSycn {
public static void main(String[] args) {
String type = "order";
String id = "1";
String key = type + id;
synchronized (key.intern()) {
//执行方法
}
}
}
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