关于java String中intern的深入讲解

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

总结

以上就是这篇文章的全部内容了，希望本文的内容对大家的学习或者工作具有一定的参考学习价值，谢谢大家对脚本之家的支持。

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