关于java String中intern的深入讲解
作者：codecraft 发布时间：2023-01-24 18:18:36
标签：java,string,intern
序
本文主要研究一下java String的intern
String.intern()
java.base/java/lang/String.java
public final class String
implements java.io.Serializable, Comparable<String>, CharSequence,
Constable, ConstantDesc {
//......
/**
* 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™ 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.
* @jls 3.10.5 String Literals
*/
public native String intern();
//......
}
当调用intern方法时，如果常量池已经包含一个equals此String对象的字符串，则返回池中的字符串
当调用intern方法时，如果常量池没有一个equals此String对象的字符串，将此String对象添加到池中，并返回此String对象的引用(即intern方法返回指向heap中的此String对象引用)
所有literal strings及string-valued constant expressions都是interned的
实例
基于jdk12
StringExistInPoolBeforeIntern
public class StringExistInPoolBeforeIntern {
public static void main(String[] args){
String stringObject = new String("tomcat");
//NOTE 在intern之前，string table已经有了tomcat，因而intern返回tomcat，不会指向stringObject
stringObject.intern();
String stringLiteral = "tomcat";
System.out.println(stringObject == stringLiteral); //false
}
}
tomcat这个literal string是interned过的，常量池没有tomcat，因而添加到常量池，常量池有个tomcat；另外由于stringObject是new的，所以heap中也有一个tomcat，而此时它指向heap中的tomcat
stringObject.intern()返回的是heap中常量池的tomcat；stringLiteral是tomcat这个literal string，由于常量池已经有该值，因而stringLiteral指向的是heap中常量池的tomcat
此时stringObject指向的是heap中的tomcat，而stringLiteral是heap中常量池的tomcat，因而二者不等，返回false
StringNotExistInPoolBeforeIntern
public class StringNotExistInPoolBeforeIntern {
public static void main(String[] args){
String stringObject = new String("tom") + new String("cat");
//NOTE 在intern之前，string table没有tomcat，因而intern指向stringObject
stringObject.intern();
String stringLiteral = "tomcat";
System.out.println(stringObject == stringLiteral); //true
}
}
tom及cat这两个literal string是interned过的，常量池没有tom及cat，因而添加到常量池，常量池有tom、cat；另外由于stringObject是new出来的，是tom及cat二者concat，因而heap中有一个tomcat
stringObject的intern方法执行的时候，由于常量池中没有tomcat，因而添加到常量池，intern()返回的是指向heap中的tomcat的引用；stringLiteral是tomcat这个literal string，由于stringObject.intern()已经将tomcat添加到常量池了并指向heap中的tomcat的引用，所以stringLiteral返回的是指向heap中的tomcat的引用
由于stringLiteral返回的是指向heap中的tomcat的引用，其实就是stringObject，因而二者相等，返回true
javap
基于jdk12
StringExistInPoolBeforeIntern
javac src/main/java/com/example/javac/StringExistInPoolBeforeIntern.java
javap -v src/main/java/com/example/javac/StringExistInPoolBeforeIntern.class
Last modified 2019年4月6日; size 683 bytes
MD5 checksum 207635ffd7560f1df24b98607e2ca7db
Compiled from "StringExistInPoolBeforeIntern.java"
public class com.example.javac.StringExistInPoolBeforeIntern
minor version: 0
major version: 56
flags: (0x0021) ACC_PUBLIC, ACC_SUPER
this_class: #8 // com/example/javac/StringExistInPoolBeforeIntern
super_class: #9 // java/lang/Object
interfaces: 0, fields: 0, methods: 2, attributes: 1
Constant pool:
#1 = Methodref #9.#21 // java/lang/Object."<init>":()V
#2 = Class #22 // java/lang/String
#3 = String #23 // tomcat
#4 = Methodref #2.#24 // java/lang/String."<init>":(Ljava/lang/String;)V
#5 = Methodref #2.#25 // java/lang/String.intern:()Ljava/lang/String;
#6 = Fieldref #26.#27 // java/lang/System.out:Ljava/io/PrintStream;
#7 = Methodref #18.#28 // java/io/PrintStream.println:(Z)V
#8 = Class #29 // com/example/javac/StringExistInPoolBeforeIntern
#9 = Class #30 // java/lang/Object
#10 = Utf8 <init>
#11 = Utf8 ()V
#12 = Utf8 Code
#13 = Utf8 LineNumberTable
#14 = Utf8 main
#15 = Utf8 ([Ljava/lang/String;)V
#16 = Utf8 StackMapTable
#17 = Class #31 // "[Ljava/lang/String;"
#18 = Class #32 // java/io/PrintStream
#19 = Utf8 SourceFile
#20 = Utf8 StringExistInPoolBeforeIntern.java
#21 = NameAndType #10:#11 // "<init>":()V
#22 = Utf8 java/lang/String
#23 = Utf8 tomcat
#24 = NameAndType #10:#33 // "<init>":(Ljava/lang/String;)V
#25 = NameAndType #34:#35 // intern:()Ljava/lang/String;
#26 = Class #36 // java/lang/System
#27 = NameAndType #37:#38 // out:Ljava/io/PrintStream;
#28 = NameAndType #39:#40 // println:(Z)V
#29 = Utf8 com/example/javac/StringExistInPoolBeforeIntern
#30 = Utf8 java/lang/Object
#31 = Utf8 [Ljava/lang/String;
#32 = Utf8 java/io/PrintStream
#33 = Utf8 (Ljava/lang/String;)V
#34 = Utf8 intern
#35 = Utf8 ()Ljava/lang/String;
#36 = Utf8 java/lang/System
#37 = Utf8 out
#38 = Utf8 Ljava/io/PrintStream;
#39 = Utf8 println
#40 = Utf8 (Z)V
{
public com.example.javac.StringExistInPoolBeforeIntern();
descriptor: ()V
flags: (0x0001) ACC_PUBLIC
Code:
stack=1, locals=1, args_size=1
0: aload_0
1: invokespecial #1 // Method java/lang/Object."<init>":()V
4: return
LineNumberTable:
line 8: 0
public static void main(java.lang.String[]);
descriptor: ([Ljava/lang/String;)V
flags: (0x0009) ACC_PUBLIC, ACC_STATIC
Code:
stack=3, locals=3, args_size=1
0: new #2 // class java/lang/String
3: dup
4: ldc #3 // String tomcat
6: invokespecial #4 // Method java/lang/String."<init>":(Ljava/lang/String;)V
9: astore_1
10: aload_1
11: invokevirtual #5 // Method java/lang/String.intern:()Ljava/lang/String;
14: pop
15: ldc #3 // String tomcat
17: astore_2
18: getstatic #6 // Field java/lang/System.out:Ljava/io/PrintStream;
21: aload_1
22: aload_2
23: if_acmpne 30
26: iconst_1
27: goto 31
30: iconst_0
31: invokevirtual #7 // Method java/io/PrintStream.println:(Z)V
34: return
LineNumberTable:
line 11: 0
line 13: 10
line 14: 15
line 15: 18
line 16: 34
StackMapTable: number_of_entries = 2
frame_type = 255 /* full_frame */
offset_delta = 30
locals = [ class "[Ljava/lang/String;", class java/lang/String, class java/lang/String ]
stack = [ class java/io/PrintStream ]
frame_type = 255 /* full_frame */
offset_delta = 0
locals = [ class "[Ljava/lang/String;", class java/lang/String, class java/lang/String ]
stack = [ class java/io/PrintStream, int ]
}
SourceFile: "StringExistInPoolBeforeIntern.java"
可以看到常量池有个tomcat
StringNotExistInPoolBeforeIntern
javac src/main/java/com/example/javac/StringNotExistInPoolBeforeIntern.java
javap -v src/main/java/com/example/javac/StringNotExistInPoolBeforeIntern.class
Last modified 2019年4月6日; size 1187 bytes
MD5 checksum 6d173f303b61b8f5826e54bb6ed5157c
Compiled from "StringNotExistInPoolBeforeIntern.java"
public class com.example.javac.StringNotExistInPoolBeforeIntern
minor version: 0
major version: 56
flags: (0x0021) ACC_PUBLIC, ACC_SUPER
this_class: #11 // com/example/javac/StringNotExistInPoolBeforeIntern
super_class: #12 // java/lang/Object
interfaces: 0, fields: 0, methods: 2, attributes: 3
Constant pool:
#1 = Methodref #12.#24 // java/lang/Object."<init>":()V
#2 = Class #25 // java/lang/String
#3 = String #26 // tom
#4 = Methodref #2.#27 // java/lang/String."<init>":(Ljava/lang/String;)V
#5 = String #28 // cat
#6 = InvokeDynamic #0:#32 // #0:makeConcatWithConstants:(Ljava/lang/String;Ljava/lang/String;)Ljava/lang/String;
#7 = Methodref #2.#33 // java/lang/String.intern:()Ljava/lang/String;
#8 = String #34 // tomcat
#9 = Fieldref #35.#36 // java/lang/System.out:Ljava/io/PrintStream;
#10 = Methodref #21.#37 // java/io/PrintStream.println:(Z)V
#11 = Class #38 // com/example/javac/StringNotExistInPoolBeforeIntern
#12 = Class #39 // java/lang/Object
#13 = Utf8 <init>
#14 = Utf8 ()V
#15 = Utf8 Code
#16 = Utf8 LineNumberTable
#17 = Utf8 main
#18 = Utf8 ([Ljava/lang/String;)V
#19 = Utf8 StackMapTable
#20 = Class #40 // "[Ljava/lang/String;"
#21 = Class #41 // java/io/PrintStream
#22 = Utf8 SourceFile
#23 = Utf8 StringNotExistInPoolBeforeIntern.java
#24 = NameAndType #13:#14 // "<init>":()V
#25 = Utf8 java/lang/String
#26 = Utf8 tom
#27 = NameAndType #13:#42 // "<init>":(Ljava/lang/String;)V
#28 = Utf8 cat
#29 = Utf8 BootstrapMethods
#30 = MethodHandle 6:#43 // REF_invokeStatic java/lang/invoke/StringConcatFactory.makeConcatWithConstants:(Ljava/lang/invoke/MethodHandles$Lookup;Ljava/lang/String;Ljava/lang/invoke/MethodType;Ljava/lang/String;[Ljava/lang/Object;)Ljava/lang/invoke/CallSite;
#31 = String #44 // \u0001\u0001
#32 = NameAndType #45:#46 // makeConcatWithConstants:(Ljava/lang/String;Ljava/lang/String;)Ljava/lang/String;
#33 = NameAndType #47:#48 // intern:()Ljava/lang/String;
#34 = Utf8 tomcat
#35 = Class #49 // java/lang/System
#36 = NameAndType #50:#51 // out:Ljava/io/PrintStream;
#37 = NameAndType #52:#53 // println:(Z)V
#38 = Utf8 com/example/javac/StringNotExistInPoolBeforeIntern
#39 = Utf8 java/lang/Object
#40 = Utf8 [Ljava/lang/String;
#41 = Utf8 java/io/PrintStream
#42 = Utf8 (Ljava/lang/String;)V
#43 = Methodref #54.#55 // java/lang/invoke/StringConcatFactory.makeConcatWithConstants:(Ljava/lang/invoke/MethodHandles$Lookup;Ljava/lang/String;Ljava/lang/invoke/MethodType;Ljava/lang/String;[Ljava/lang/Object;)Ljava/lang/invoke/CallSite;
#44 = Utf8 \u0001\u0001
#45 = Utf8 makeConcatWithConstants
#46 = Utf8 (Ljava/lang/String;Ljava/lang/String;)Ljava/lang/String;
#47 = Utf8 intern
#48 = Utf8 ()Ljava/lang/String;
#49 = Utf8 java/lang/System
#50 = Utf8 out
#51 = Utf8 Ljava/io/PrintStream;
#52 = Utf8 println
#53 = Utf8 (Z)V
#54 = Class #56 // java/lang/invoke/StringConcatFactory
#55 = NameAndType #45:#60 // makeConcatWithConstants:(Ljava/lang/invoke/MethodHandles$Lookup;Ljava/lang/String;Ljava/lang/invoke/MethodType;Ljava/lang/String;[Ljava/lang/Object;)Ljava/lang/invoke/CallSite;
#56 = Utf8 java/lang/invoke/StringConcatFactory
#57 = Class #62 // java/lang/invoke/MethodHandles$Lookup
#58 = Utf8 Lookup
#59 = Utf8 InnerClasses
#60 = Utf8 (Ljava/lang/invoke/MethodHandles$Lookup;Ljava/lang/String;Ljava/lang/invoke/MethodType;Ljava/lang/String;[Ljava/lang/Object;)Ljava/lang/invoke/CallSite;
#61 = Class #63 // java/lang/invoke/MethodHandles
#62 = Utf8 java/lang/invoke/MethodHandles$Lookup
#63 = Utf8 java/lang/invoke/MethodHandles
{
public com.example.javac.StringNotExistInPoolBeforeIntern();
descriptor: ()V
flags: (0x0001) ACC_PUBLIC
Code:
stack=1, locals=1, args_size=1
0: aload_0
1: invokespecial #1 // Method java/lang/Object."<init>":()V
4: return
LineNumberTable:
line 8: 0
public static void main(java.lang.String[]);
descriptor: ([Ljava/lang/String;)V
flags: (0x0009) ACC_PUBLIC, ACC_STATIC
Code:
stack=4, locals=3, args_size=1
0: new #2 // class java/lang/String
3: dup
4: ldc #3 // String tom
6: invokespecial #4 // Method java/lang/String."<init>":(Ljava/lang/String;)V
9: new #2 // class java/lang/String
12: dup
13: ldc #5 // String cat
15: invokespecial #4 // Method java/lang/String."<init>":(Ljava/lang/String;)V
18: invokedynamic #6, 0 // InvokeDynamic #0:makeConcatWithConstants:(Ljava/lang/String;Ljava/lang/String;)Ljava/lang/String;
23: astore_1
24: aload_1
25: invokevirtual #7 // Method java/lang/String.intern:()Ljava/lang/String;
28: pop
29: ldc #8 // String tomcat
31: astore_2
32: getstatic #9 // Field java/lang/System.out:Ljava/io/PrintStream;
35: aload_1
36: aload_2
37: if_acmpne 44
40: iconst_1
41: goto 45
44: iconst_0
45: invokevirtual #10 // Method java/io/PrintStream.println:(Z)V
48: return
LineNumberTable:
line 11: 0
line 13: 24
line 14: 29
line 15: 32
line 16: 48
StackMapTable: number_of_entries = 2
frame_type = 255 /* full_frame */
offset_delta = 44
locals = [ class "[Ljava/lang/String;", class java/lang/String, class java/lang/String ]
stack = [ class java/io/PrintStream ]
frame_type = 255 /* full_frame */
offset_delta = 0
locals = [ class "[Ljava/lang/String;", class java/lang/String, class java/lang/String ]
stack = [ class java/io/PrintStream, int ]
}
SourceFile: "StringNotExistInPoolBeforeIntern.java"
InnerClasses:
public static final #58= #57 of #61; // Lookup=class java/lang/invoke/MethodHandles$Lookup of class java/lang/invoke/MethodHandles
BootstrapMethods:
0: #30 REF_invokeStatic java/lang/invoke/StringConcatFactory.makeConcatWithConstants:(Ljava/lang/invoke/MethodHandles$Lookup;Ljava/lang/String;Ljava/lang/invoke/MethodType;Ljava/lang/String;[Ljava/lang/Object;)Ljava/lang/invoke/CallSite;
Method arguments:
#31 \u0001\u0001
可以看到常量池有tom、cat、tomcat
小结
当调用intern方法时，如果常量池已经包含一个equals此String对象的字符串，则返回池中的字符串
当调用intern方法时，如果常量池没有一个equals此String对象的字符串，将此String对象添加到池中，并返回此String对象的引用(即intern方法返回指向heap中的此String对象引用)
所有literal strings及string-valued constant expressions都是interned的
来源：https://segmentfault.com/a/1190000018775484
0
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