Why should string be designed to be immutable?

interpreter : Anchor

Translation date : 2013 year 11 month 18 Japan

Link to the original text : ​ ​Why string is immutable in Java ?​​

It's a platitude (This is an old yet still popular question). stay Java Lieutenant general String Designed to be immutable is the result of taking into account various factors , To understand this question , Need integrated memory , Sync , Data structure and security considerations . In the following , I will make a summary for various reasons .

1. The need for string constant pool

String constant pool (String pool, String intern pool, String Keep the pool ) yes Java A special storage area in heap memory , When creating a String Object time , If the string value already exists in the constant pool , A new object will not be created , It's a reference to an existing object .

This is shown in the following code , Only one actual... Will be created in heap memory String object .

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        String s1 = "abcd";
        
String s2 = "abcd";
       
       

        
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The schematic diagram is shown below :

chart 1

If the string object is allowed to change , Then it will lead to all kinds of logical errors , For example, changing one object can affect another independent object . Strictly speaking , This idea of constant pool , It's a means of optimization .

please reflection : Suppose the code looks like this ,s1 and s2 It will also point to the same actual String Objects? ?

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        String s1= "ab" + "cd";
        
String s2= "abc" + "d";
       
       

        
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Maybe this question goes against the novice's intuition , But considering that modern compilers do regular optimizations , So they all point to the same object in the constant pool . perhaps , You can use it.

jd-gui Look at the compiled class file .

2. allow String Object caching HashCode

Java in String The hash code of the object is frequently used , For example hashMap In a container .

String invariance guarantees hash Uniqueness of codes , So it's safe to cache . It's also a performance optimization tool , That means you don't have to compute a new hash code every time . stay String Class definition has the following code :

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        private int hash;// Used to cache HashCode
       
       

        
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3. Security

String By many Java class ( library ) Used as a parameter , for example Network connection address URL, File path path, And what reflection mechanisms need String Parameters, etc. , If String It's not fixed , Will cause all kinds of security risks .

Suppose you have the following code :

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        boolean connect(string s){
        
    if (!isSecure(s)) { 
        
throw new SecurityException(); 
        
}
        
    //  If it can be modified elsewhere String, Then there will be all kinds of unexpected problems / error  
        
    causeProblem(s);
        
}
       
       

        
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On the whole , String The reasons for immutability include Design considerations , Efficiency optimization problem , And security . in fact , This is also Java Many of the interviews " Why? " The answer .

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