关于java String中intern的深入讲解

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();

  //......  

}

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

  }

}

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

  }

}

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"

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