理解String 及 String.intern() 在实际中的应用

Java代码



1. 首先String不属于8种基本数据类型，String是一个对象。

　　因为对象的默认值是null，所以String的默认值也是null；但它又是一种特殊的对象，有其它对象没有的一些特性。

　　2. new String()和new String(“”)都是申明一个新的空字符串，是空串不是null；

　　3. String str=”kvill”；

String str=new String (“kvill”);的区别：

　　在这里，我们不谈堆，也不谈栈，只先简单引入常量池这个简单的概念。

　　常量池(constant pool)指的是在编译期被确定，并被保存在已编译的.class文件中的一些数据。它包括了关于类、方法、接口等中的常量，也包括字符串常量。

　　看例1：

String s0=”kvill”;

String s1=”kvill”;

String s2=”kv” + “ill”;

System.out.println( s0==s1 );

System.out.println( s0==s2 );

　　结果为：

true

true

　　首先，我们要知道Java会确保一个字符串常量只有一个拷贝。

　　因为例子中的s0和s1中的”kvill”都是字符串常量，它们在编译期就被确定了，所以s0==s1为true；而”kv”和”ill”也都是字符串常量，当一个字符串由多个字符串常量连接而成时，它自己肯定也是字符串常量，所以s2也同样在编译期就被解析为一个字符串常量，所以s2也是常量池中”kvill”的一个引用。

　　所以我们得出s0==s1==s2;

　　用new String() 创建的字符串不是常量，不能在编译期就确定，所以new String() 创建的字符串不放入常量池中，它们有自己的地址空间。

　　看例2：

String s0=”kvill”;

String s1=new String(”kvill”);

String s2=”kv” + new String(“ill”);

System.out.println( s0==s1 );

System.out.println( s0==s2 );

System.out.println( s1==s2 );

　　结果为：

false

false

false

　　例2中s0还是常量池中”kvill”的应用，s1因为无法在编译期确定，所以是运行时创建的新对象”kvill”的引用，s2因为有后半部分new String(“ill”)所以也无法在编译期确定，所以也是一个新创建对象”kvill”的应用;明白了这些也就知道为何得出此结果了。

　　4. String.intern()：

　　再补充介绍一点：存在于.class文件中的常量池，在运行期被JVM装载，并且可以扩充。String的intern()方法就是扩充常量池的一个方法；当一个String实例str调用intern()方法时，Java查找常量池中是否有相同Unicode的字符串常量，如果有，则返回其的引用，如果没有，则在常量池中增加一个Unicode等于str的字符串并返回它的引用；看例3就清楚了

　　例3：

String s0= “kvill”;

String s1=new String(”kvill”);

String s2=new String(“kvill”);

System.out.println( s0==s1 );

System.out.println( “**********” );

s1.intern();

s2=s2.intern(); //把常量池中“kvill”的引用赋给s2

System.out.println( s0==s1);

System.out.println( s0==s1.intern() );

System.out.println( s0==s2 );

　　结果为：

false

**********

false //虽然执行了s1.intern(),但它的返回值没有赋给s1

true //说明s1.intern()返回的是常量池中”kvill”的引用

true

　　最后我再破除一个错误的理解：

　　有人说，“使用String.intern()方法则可以将一个String类的保存到一个全局String表中，如果具有相同值的Unicode字符串已经在这个表中，那么该方法返回表中已有字符串的地址，如果在表中没有相同值的字符串，则将自己的地址注册到表中“如果我把他说的这个全局的String表理解为常量池的话，他的最后一句话，“如果在表中没有相同值的字符串，则将自己的地址注册到表中”是错的：

　　看例4：

String s1=new String("kvill");

String s2=s1.intern();

System.out.println( s1==s1.intern() );

System.out.println( s1+" "+s2 );

System.out.println( s2==s1.intern() );

　　结果：

false

kvill kvill

true

　　在这个类中我们没有声名一个”kvill”常量，所以常量池中一开始是没有”kvill”的，当我们调用s1.intern()后就在常量池中新添加了一个”kvill”常量，原来的不在常量池中的”kvill”仍然存在，也就不是“将自己的地址注册到常量池中”了。

　　s1==s1.intern()为false说明原来的“kvill”仍然存在；

　　s2现在为常量池中“kvill”的地址，所以有s2==s1.intern()为true。

　　5. 关于equals()和==:

　　这个对于String简单来说就是比较两字符串的Unicode序列是否相当，如果相等返回true;而==是比较两字符串的地址是否相同，也就是是否是同一个字符串的引用。

　　6. 关于String是不可变的

　　这一说又要说很多，大家只要知道String的实例一旦生成就不会再改变了，比如说：String str=”kv”+”ill”+” “+”ans”;

就是有4个字符串常量，首先”kv”和”ill”生成了”kvill”存在内存中，然后”kvill”又和” “ 生成 ”kvill “存在内存中，最后又和生成了”kvill ans”;并把这个字符串的地址赋给了str,就是因为String的“不可变”产生了很多临时变量，这也就是为什么建议用StringBuffer的原因了，因为StringBuffer是可改变的

出处：http://www.iteye.com/topic/122206

By the way,关于 String.intern() 在实际中的应用，我在tomcat的源码中找到了一个地方用到了，如下：

Java代码



/*

* Copyright 1999,2004-2005 The Apache Software Foundation.

*

* Licensed under the Apache License, Version 2.0 (the "License");

* you may not use this file except in compliance with the License.

* You may obtain a copy of the License at

*

* http://www.apache.org/licenses/LICENSE-2.0
*

* Unless required by applicable law or agreed to in writing, software

* distributed under the License is distributed on an "AS IS" BASIS,

* WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.

* See the License for the specific language governing permissions and

* limitations under the License.

* ====================================================================

*

* This software consists of voluntary contributions made by many

* individuals on behalf of the Apache Software Foundation and was

* originally based on software copyright (c) 1999, International

* Business Machines, Inc., http://www.apache.org. For more

* information on the Apache Software Foundation, please see

* <http://www.apache.org/>.

*/

package org.apache.jasper.xmlparser;

/**

* This class is a symbol table implementation that guarantees that

* strings used as identifiers are unique references. Multiple calls

* to <code>addSymbol</code> will always return the same string

* reference.

* <p>

* The symbol table performs the same task as <code>String.intern()</code>

* with the following differences:

* <ul>

* <li>

* A new string object does not need to be created in order to

* retrieve a unique reference. Symbols can be added by using

* a series of characters in a character array.

* </li>

* <li>

* Users of the symbol table can provide their own symbol hashing

* implementation. For example, a simple string hashing algorithm

* may fail to produce a balanced set of hashcodes for symbols

* that are <em>mostly</em> unique. Strings with similar leading

* characters are especially prone to this poor hashing behavior.

* </li>

* </ul>

*

* @author Andy Clark

* @version $Id: SymbolTable.java 306179 2005-07-27 15:12:04Z yoavs $

*/

public class SymbolTable {

//

// Constants

//

/** Default table size. */

protected static final int TABLE_SIZE = 101;

//

// Data

//

/** Buckets. */

protected Entry[] fBuckets = null;

// actual table size

protected int fTableSize;

//

// Constructors

//

/** Constructs a symbol table with a default number of buckets. */

public SymbolTable() {

this(TABLE_SIZE);

}

/** Constructs a symbol table with a specified number of buckets. */

public SymbolTable(int tableSize) {

fTableSize = tableSize;

fBuckets = new Entry[fTableSize];

}

//

// Public methods

//

/**

* Adds the specified symbol to the symbol table and returns a

* reference to the unique symbol. If the symbol already exists,

* the previous symbol reference is returned instead, in order

* guarantee that symbol references remain unique.

*

* @param symbol The new symbol.

*/

public String addSymbol(String symbol) {

// search for identical symbol

int bucket = hash(symbol) % fTableSize;

int length = symbol.length();

OUTER: for (Entry entry = fBuckets[bucket]; entry != null; entry = entry.next) {

if (length == entry.characters.length) {

for (int i = 0; i < length; i++) {

if (symbol.charAt(i) != entry.characters[i]) {

continue OUTER;

}

}

return entry.symbol;

}

}

// create new entry

Entry entry = new Entry(symbol, fBuckets[bucket]);

fBuckets[bucket] = entry;

return entry.symbol;

} // addSymbol(String):String

/**

* Adds the specified symbol to the symbol table and returns a

* reference to the unique symbol. If the symbol already exists,

* the previous symbol reference is returned instead, in order

* guarantee that symbol references remain unique.

*

* @param buffer The buffer containing the new symbol.

* @param offset The offset into the buffer of the new symbol.

* @param length The length of the new symbol in the buffer.

*/

public String addSymbol(char[] buffer, int offset, int length) {

// search for identical symbol

int bucket = hash(buffer, offset, length) % fTableSize;

OUTER: for (Entry entry = fBuckets[bucket]; entry != null; entry = entry.next) {

if (length == entry.characters.length) {

for (int i = 0; i < length; i++) {

if (buffer[offset + i] != entry.characters[i]) {

continue OUTER;

}

}

return entry.symbol;

}

}

// add new entry

Entry entry = new Entry(buffer, offset, length, fBuckets[bucket]);

fBuckets[bucket] = entry;

return entry.symbol;

} // addSymbol(char[],int,int):String

/**

* Returns a hashcode value for the specified symbol. The value

* returned by this method must be identical to the value returned

* by the <code>hash(char[],int,int)</code> method when called

* with the character array that comprises the symbol string.

*

* @param symbol The symbol to hash.

*/

public int hash(String symbol) {

int code = 0;

int length = symbol.length();

for (int i = 0; i < length; i++) {

code = code * 37 + symbol.charAt(i);

}

return code & 0x7FFFFFF;

} // hash(String):int

/**

* Returns a hashcode value for the specified symbol information.

* The value returned by this method must be identical to the value

* returned by the <code>hash(String)</code> method when called

* with the string object created from the symbol information.

*

* @param buffer The character buffer containing the symbol.

* @param offset The offset into the character buffer of the start

* of the symbol.

* @param length The length of the symbol.

*/

public int hash(char[] buffer, int offset, int length) {

int code = 0;

for (int i = 0; i < length; i++) {

code = code * 37 + buffer[offset + i];

}

return code & 0x7FFFFFF;

} // hash(char[],int,int):int

/**

* Returns true if the symbol table already contains the specified

* symbol.

*

* @param symbol The symbol to look for.

*/

public boolean containsSymbol(String symbol) {

// search for identical symbol

int bucket = hash(symbol) % fTableSize;

int length = symbol.length();

OUTER: for (Entry entry = fBuckets[bucket]; entry != null; entry = entry.next) {

if (length == entry.characters.length) {

for (int i = 0; i < length; i++) {

if (symbol.charAt(i) != entry.characters[i]) {

continue OUTER;

}

}

return true;

}

}

return false;

} // containsSymbol(String):boolean

/**

* Returns true if the symbol table already contains the specified

* symbol.

*

* @param buffer The buffer containing the symbol to look for.

* @param offset The offset into the buffer.

* @param length The length of the symbol in the buffer.

*/

public boolean containsSymbol(char[] buffer, int offset, int length) {

// search for identical symbol

int bucket = hash(buffer, offset, length) % fTableSize;

OUTER: for (Entry entry = fBuckets[bucket]; entry != null; entry = entry.next) {

if (length == entry.characters.length) {

for (int i = 0; i < length; i++) {

if (buffer[offset + i] != entry.characters[i]) {

continue OUTER;

}

}

return true;

}

}

return false;

} // containsSymbol(char[],int,int):boolean

//

// Classes

//

/**

* This class is a symbol table entry. Each entry acts as a node

* in a linked list.

*/

protected static final class Entry {

//

// Data

//

/** Symbol. */

public String symbol;

/**

* Symbol characters. This information is duplicated here for

* comparison performance.

*/

public char[] characters;

/** The next entry. */

public Entry next;

//

// Constructors

//

/**

* Constructs a new entry from the specified symbol and next entry

* reference.

*/

public Entry(String symbol, Entry next) {

this.symbol = symbol.intern();

characters = new char[symbol.length()];

symbol.getChars(0, characters.length, characters, 0);

this.next = next;

}

/**

* Constructs a new entry from the specified symbol information and

* next entry reference.

*/

public Entry(char[] ch, int offset, int length, Entry next) {

characters = new char[length];

System.arraycopy(ch, offset, characters, 0, length);

symbol = new String(characters).intern();

this.next = next;

}

} // class Entry

} // class SymbolTable