In depth explanation of intern in java String

2021-07-13 05:24:04
OfStack

Sequence

In this paper, we mainly study the intern of java String under 1

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&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.
  * @jls 3.10.5 String Literals
  */
 public native String intern();

 //......   

}

When the intern method is called, if the constant pool already contains a string for the equals String object, the string in the pool is returned When the intern method is called, if the constant pool does not have a string for the equals String object, add the String object to the pool and return a reference to the String object (that is, the intern method returns a reference to the String object in heap) All literal strings and string-valued constant expressions are interned

Instances

Based on jdk12

StringExistInPoolBeforeIntern


public class StringExistInPoolBeforeIntern {

 public static void main(String[] args){
  String stringObject = new String("tomcat");
  //NOTE  In intern Before, string table Already have tomcat , therefore intern Return tomcat , does not point to stringObject
  stringObject.intern();
  String stringLiteral = "tomcat";
  System.out.println(stringObject == stringLiteral); //false
 }
}

tomcat This literal string is interned, and the constant pool does not have tomcat, so it is added to the constant pool, which has an tomcat;; In addition, since stringObject is new, there is also an tomcat in heap, and at this time it points to tomcat in heap stringObject. intern () returns the tomcat of the constant pool in heap; stringLiteral is tomcat, literal, string. Since the constant pool already has this value, stringLiteral points to tomcat of the constant pool in heap At this time, stringObject points to tomcat in heap, while stringLiteral is tomcat of constant pool in heap, so the two are different and return false

StringNotExistInPoolBeforeIntern


public class StringNotExistInPoolBeforeIntern {

 public static void main(String[] args){
  String stringObject = new String("tom") + new String("cat");
  //NOTE  In intern Before, string table No tomcat , therefore intern Point stringObject
  stringObject.intern();
  String stringLiteral = "tomcat";
  System.out.println(stringObject == stringLiteral); //true
 }
}

tom and cat, two literal string, are interned, and the constant pool does not have tom and cat, so it is added to the constant pool, which includes tom and cat; In addition, because stringObject comes from new and is concat of tom and cat2, there is one tomcat in heap When the intern method of stringObject is executed, tomcat is added to the constant pool because there is no tomcat in the constant pool, and intern () returns a reference to tomcat in heap; stringLiteral is tomcat, literal string. Since stringObject. intern () has added tomcat to the constant pool and points to a reference to tomcat in heap, stringLiteral returns a reference to tomcat in heap Since stringLiteral returns a reference to tomcat in heap, which is actually stringObject, the two are equal and return true

javap

Based on jdk12

StringExistInPoolBeforeIntern


javac src/main/java/com/example/javac/StringExistInPoolBeforeIntern.java
javap -v src/main/java/com/example/javac/StringExistInPoolBeforeIntern.class

 Last modified 2019 Year 4 Month 6 Day ; 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"

You can see that the constant pool has an tomcat

StringNotExistInPoolBeforeIntern


javac src/main/java/com/example/javac/StringNotExistInPoolBeforeIntern.java
javap -v src/main/java/com/example/javac/StringNotExistInPoolBeforeIntern.class

 Last modified 2019 Year 4 Month 6 Day ; 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

You can see that the constant pools are tom, cat and tomcat

Summary

When the intern method is called, if the constant pool already contains 1 equals string for this String object, the string in the pool is returned
When the intern method is called, if the constant pool does not have a string for the equals String object, add the String object to the pool and return a reference to the String object (that is, the intern method returns a reference to the String object in heap)

All literal strings and string-valued constant expressions are interned

Summarize