序
string.intern()
java.base/java/lang/string.java
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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
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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
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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
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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
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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,谢谢大家对服务器之家的支持。
原文链接:https://segmentfault.com/a/1190000018775484