StringBuffer和StringBuilder区别？

1. String是不可变类，改变String变量中的值，相当于开辟了新的空间存放新的string变量

2. StringBuffer 可变的类，可以通过append方法改变变量的值，且StringBuffer是线程安全的，它的很多方法都是同步方法，支持并发操作，适用于多线程　　　　

3. StringBuilder 可变的类，但是线程不安全的，用于单线程中性能高于StringBuffer

4. HashTable 线程安全的，HashMap线程不安全的

为什么StringBuffer是同步的？

因为很多方法都是synchronized 。。。

/*
 * @(#)StringBuffer.java    1.101 05/11/17
 *
 * Copyright 2006 Sun Microsystems, Inc. All rights reserved.
 * SUN PROPRIETARY/CONFIDENTIAL. Use is subject to license terms.
 */

package java.lang;


/**
 * A thread-safe, mutable sequence of characters. 
 * A string buffer is like a {@link String}, but can be modified. At any 
 * point in time it contains some particular sequence of characters, but 
 * the length and content of the sequence can be changed through certain 
 * method calls.
 * <p>
 * String buffers are safe for use by multiple threads. The methods 
 * are synchronized where necessary so that all the operations on any 
 * particular instance behave as if they occur in some serial order 
 * that is consistent with the order of the method calls made by each of 
 * the individual threads involved.
 * <p>
 * The principal operations on a <code>StringBuffer</code> are the 
 * <code>append</code> and <code>insert</code> methods, which are 
 * overloaded so as to accept data of any type. Each effectively 
 * converts a given datum to a string and then appends or inserts the 
 * characters of that string to the string buffer. The 
 * <code>append</code> method always adds these characters at the end 
 * of the buffer; the <code>insert</code> method adds the characters at 
 * a specified point. 
 * <p>
 * For example, if <code>z</code> refers to a string buffer object 
 * whose current contents are "<code>start</code>", then 
 * the method call <code>z.append("le")</code> would cause the string 
 * buffer to contain "<code>startle</code>", whereas 
 * <code>z.insert(4, "le")</code> would alter the string buffer to 
 * contain "<code>starlet</code>". 
 * <p>
 * In general, if sb refers to an instance of a <code>StringBuffer</code>, 
 * then <code>sb.append(x)</code> has the same effect as 
 * <code>sb.insert(sb.length(), x)</code>.
 * <p>
 * Whenever an operation occurs involving a source sequence (such as
 * appending or inserting from a source sequence) this class synchronizes
 * only on the string buffer performing the operation, not on the source.
 * <p>
 * Every string buffer has a capacity. As long as the length of the 
 * character sequence contained in the string buffer does not exceed 
 * the capacity, it is not necessary to allocate a new internal 
 * buffer array. If the internal buffer overflows, it is 
 * automatically made larger. 
 *
 * As of  release JDK 5, this class has been supplemented with an equivalent 
 * class designed for use by a single thread, {@link StringBuilder}.  The 
 * <tt>StringBuilder</tt> class should generally be used in preference to 
 * this one, as it supports all of the same operations but it is faster, as 
 * it performs no synchronization.
 *
 * @author    Arthur van Hoff
 * @version     1.101, 11/17/05
 * @see     java.lang.StringBuilder
 * @see     java.lang.String
 * @since   JDK1.0
 */
 public final class StringBuffer
    extends AbstractStringBuilder
    implements java.io.Serializable, CharSequence
{

    /** use serialVersionUID from JDK 1.0.2 for interoperability */
    static final long serialVersionUID = 3388685877147921107L;

    /**
     * Constructs a string buffer with no characters in it and an 
     * initial capacity of 16 characters. 
     */
    public StringBuffer() {
    super(16);
    }

    /**
     * Constructs a string buffer with no characters in it and 
     * the specified initial capacity. 
     *
     * @param      capacity  the initial capacity.
     * @exception  NegativeArraySizeException  if the <code>capacity</code>
     *               argument is less than <code>0</code>.
     */
    public StringBuffer(int capacity) {
    super(capacity);
    }

    /**
     * Constructs a string buffer initialized to the contents of the 
     * specified string. The initial capacity of the string buffer is 
     * <code>16</code> plus the length of the string argument.
     *
     * @param   str   the initial contents of the buffer.
     * @exception NullPointerException if <code>str</code> is <code>null</code>
     */
    public StringBuffer(String str) {
    super(str.length() + 16);
    append(str);
    }

    /**
     * Constructs a string buffer that contains the same characters
     * as the specified <code>CharSequence</code>. The initial capacity of
     * the string buffer is <code>16</code> plus the length of the
     * <code>CharSequence</code> argument.
     * <p>
     * If the length of the specified <code>CharSequence</code> is
     * less than or equal to zero, then an empty buffer of capacity
     * <code>16</code> is returned.
     *
     * @param      seq   the sequence to copy.
     * @exception NullPointerException if <code>seq</code> is <code>null</code>
     * @since 1.5
     */
    public StringBuffer(CharSequence seq) {
        this(seq.length() + 16);
        append(seq);
    }

    public synchronized int length() {
    return count;
    }

    public synchronized int capacity() {
    return value.length;
    }


    ...

既然StringBuffer是线程同步的，那么代价就是损失性能，摘自一个测试http://blog.sina.com.cn/s/blog_5749ead90100b7lq.html

public class TestBufferAndBuilder {
 public static void main(String[] args) {

  String randoms[] = { String.valueOf(Math.random()), String.valueOf(Math.random()), String.valueOf(Math.random()), String.valueOf(Math.random()),
    String.valueOf(Math.random()), String.valueOf(Math.random()), String.valueOf(Math.random()), String.valueOf(Math.random()), String.valueOf(Math.random()),
    String.valueOf(Math.random()) };

  System.gc();
  long d = System.currentTimeMillis();
  for (int i = 0; i < 1000000; i++) {
   StringBuffer buffer = new StringBuffer();
   buffer.append(randoms[0]).append(randoms[1]).append(randoms[2]).append(randoms[3]).append(randoms[4]).append(randoms[5]).append(randoms[6]).append(randoms[7]).append(
     randoms[8]).append(randoms[9]);
  }
  System.err.println(System.currentTimeMillis() - d);
  System.gc();

  d = System.currentTimeMillis();
  for (int i = 0; i < 1000000; i++) {
   StringBuilder builder = new StringBuilder();
   builder.append(randoms[0]).append(randoms[1]).append(randoms[2]).append(randoms[3]).append(randoms[4]).append(randoms[5]).append(randoms[6]).append(randoms[7]).append(
     randoms[8]).append(randoms[9]);
  }
  System.err.println(System.currentTimeMillis() - d);

  System.gc();
  d = System.currentTimeMillis();
  for (int i = 0; i < 1000000; i++) {
   StringBuffer buffer = new StringBuffer(200);
   buffer.append(randoms[0]).append(randoms[1]).append(randoms[2]).append(randoms[3]).append(randoms[4]).append(randoms[5]).append(randoms[6]).append(randoms[7]).append(
     randoms[8]).append(randoms[9]);
  }
  System.err.println(System.currentTimeMillis() - d);
  System.gc();

  d = System.currentTimeMillis();
  for (int i = 0; i < 1000000; i++) {
   StringBuilder builder = new StringBuilder(200);
   builder.append(randoms[0]).append(randoms[1]).append(randoms[2]).append(randoms[3]).append(randoms[4]).append(randoms[5]).append(randoms[6]).append(randoms[7]).append(
     randoms[8]).append(randoms[9]);
  }
  System.err.println(System.currentTimeMillis() - d);

 }
}

输出结果：

    3188
    3657
    1672
    1859

结果如上，基本上要慢10-20%