經常有關String，StringBuffer,StringBuilder之間比較的論調出現，結論通常是單執行緒情況下StringBuilder效能優於StringBuffer,但StringBuilder是執行緒不安全的，所以多執行緒併發程式設計時候要用StringBuffer。執行緒安全就是說多執行緒訪問同一程式碼，不會產生不確定的結果。編寫執行緒安全的程式碼是低靠執行緒同步。通常情況下，執行緒不安全的類效率高，速度更快 。那麼，StringBuffer是如何保證執行緒安全的呢？
來吧，不道聽途說，翻一翻原始碼。

 * @author      Arthur van Hoff
 * @see
 
     java.lang.StringBuilder
 * @see     java.lang.String
 * @since   JDK1.0
 */
public final class StringBuffer   extends AbstractStringBuilder implements java.io.Serializable, CharSequence

 * @author      Michael McCloskey
 * @see         java.lang.StringBuffer
 * @see         java.lang.String
 * @since
 
       1.5
 */
public final class StringBuilder  extends AbstractStringBuilder implements java.io.Serializable, CharSequence

這裡可見StringBuffer和StringBuilder來源出處是一致的，繼承相同的類，實現相同的介面。這裡可以看出StringBuffer從JDK1.0開始就有，StringBuilder從JDK1.5才出現。像大家所知道的那樣，StringBuilder是為了提升StringBuffer效率而出現的。

StringBuffer的構造方法：

 /**
     * 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);
    }

StringBuilder的構造方法：

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

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

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

    /**
     * Constructs a string builder that contains the same characters
     * as the specified <code>CharSequence</code>. The initial capacity of
     * the string builder is <code>16</code> plus the length of the
     * <code>CharSequence</code> argument.
     *
     * @param      seq   the sequence to copy.
     * @throws    NullPointerException if <code>seq</code> is <code>null</code>
     */
    public StringBuilder(CharSequence seq) {
        this(seq.length() + 16);
        append(seq);
    }

所有構造方法都一致。

public synchronized int length() {
        return count;
    }

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


    public synchronized void ensureCapacity(int minimumCapacity) {
        if (minimumCapacity > value.length) {
            expandCapacity(minimumCapacity);
        }
    }

StringBuilder沒有重寫length()和capacity()等方法，StringBuffer給方法添加了同步鎖，從這裡開始，StringBuffer和StringBuilder出現區別。StringBuffer把所有操作類的方法均新增同步鎖，而StringBuilder並沒有。

StringBuffer的append方法：

    public synchronized StringBuffer append(Object obj) {
        super.append(String.valueOf(obj));
        return this;
    }

    public synchronized StringBuffer append(String str) {
        super.append(str);
        return this;
    }

StringBuilder的append方法：

 private StringBuilder append(StringBuilder sb) {
        if (sb == null)
            return append("null");
        int len = sb.length();
        int newcount = count + len;
        if (newcount > value.length)
            expandCapacity(newcount);
        sb.getChars(0, len, value, count);
        count = newcount;
        return this;
    }

這裡append，作為StringBuffer/StringBuilder最常用的方法之一，區別已經很明顯了。（這裡也可以看到，StringBuffer的很多方法是在JDK1.5版本的時候才新增的）
檢視synchronized的描述吧：
synchronized 方法控制對類成員變數的訪問：每個類例項對應一把鎖，每個 synchronized 方法都必須 獲得呼叫該方法的類例項的鎖方能執行，否則所屬執行緒阻塞，方法一旦執行，就獨佔該鎖，直到從該方法 返回時才將鎖釋放，此後被阻塞的執行緒方能獲得該鎖，重新進入可執行狀態。
所以，StringBuffer不可避免的比StringBuilder慢。在不需要考慮執行緒安全的程式碼，儘量還是使用StringBuilder吧。順便，HashTable和HashMap之間的關係也是這樣的。HashTable原始碼裡，也充斥著synchronized。