关于java String中intern的深入讲解
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2024-02-22 19:02:12
序
本文主要研究一下java string的intern
string.intern()
java.base/java/lang/string.java...
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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