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

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