理解String 及 String.intern() 在实际中的应用

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：

 

/*
 * 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