Java 字符串效率拼接测试报告—StringBuffer效率之王

目录

实验报告

 测试代码

StringBuffer

StringBuilder

如何推导结论？

---

从https://www.cnblogs.com/twzheng/p/5923642.html一文的结论进行验证。

发现效率StringBuilder和StringBuffer的结果是相反的，故此本文加以论证。 

实验报告

StringBuffer>StringBuilder>StringJoin>StringContact>StringPlus+

 测试代码

用其原文测试代码：

public class StringUtilsTests {
 
    private static final int max = 1000000;
 
    @Test
    public void testPlus() {
        System.out.println(">>> testPlus() <<<");
 
        String str = "";
 
        long start = System.currentTimeMillis();
 
        for (int i = 0; i < max; i++) {
            str = str + "a";
        }
 
        long end = System.currentTimeMillis();
 
        long cost = end - start;
 
        System.out.println("   {str + \"a\"} cost=" + cost + " ms");
    }
 
    @Test
    public void testConcat() {
        System.out.println(">>> testConcat() <<<");
 
        String str = "";
 
        long start = System.currentTimeMillis();
 
        for (int i = 0; i < max; i++) {
            str = str.concat("a");
        }
 
        long end = System.currentTimeMillis();
 
        long cost = end - start;
 
        System.out.println("   {str.concat(\"a\")} cost=" + cost + " ms");
    }
 
    @Test
    public void testJoin() {
        System.out.println(">>> testJoin() <<<");
 
        long start = System.currentTimeMillis();
 
        List<String> list = new ArrayList<String>();
 
        for (int i = 0; i < max; i++) {
            list.add("a");
        }
 
        long end1 = System.currentTimeMillis();
        long cost1 = end1 - start;
 
        StringUtils.join(list, "");
 
        long end = System.currentTimeMillis();
        long cost = end - end1;
 
        System.out.println("   {list.add(\"a\")} cost1=" + cost1 + " ms");
        System.out.println("   {StringUtils.join(list, \"\")} cost=" + cost
            + " ms");
    }
 
    @Test
    public void testStringBuffer() {
        System.out.println(">>> testStringBuffer() <<<");
 
        long start = System.currentTimeMillis();
 
        StringBuffer strBuffer = new StringBuffer();
 
        for (int i = 0; i < max; i++) {
            strBuffer.append("a");
        }
        strBuffer.toString();
 
        long end = System.currentTimeMillis();
 
        long cost = end - start;
 
        System.out.println("   {strBuffer.append(\"a\")} cost=" + cost + " ms");
    }
 
    @Test
    public void testStringBuilder() {
        System.out.println(">>> testStringBuilder() <<<");
 
        long start = System.currentTimeMillis();
 
        StringBuilder strBuilder = new StringBuilder();
 
        for (int i = 0; i < max; i++) {
            strBuilder.append("a");
        }
        strBuilder.toString();
 
        long end = System.currentTimeMillis();
 
        long cost = end - start;
 
        System.out
            .println("   {strBuilder.append(\"a\")} cost=" + cost + " ms");
    }
 
 
 
}

StringBuffer

我们重点看append的实现调用的是AbstractStringBuilder类方法：

 @Override
    public AbstractStringBuilder append(CharSequence s) {
        if (s == null)
            return appendNull();
        if (s instanceof String)
            return this.append((String)s);
        if (s instanceof AbstractStringBuilder)
            return this.append((AbstractStringBuilder)s);
 
        return this.append(s, 0, s.length());
    }
 

最终调用的是String的字符数组拷贝机制：

 /**
     * Copies characters from this string into the destination character
     * array.
     * <p>
     * The first character to be copied is at index {@code srcBegin};
     * the last character to be copied is at index {@code srcEnd-1}
     * (thus the total number of characters to be copied is
     * {@code srcEnd-srcBegin}). The characters are copied into the
     * subarray of {@code dst} starting at index {@code dstBegin}
     * and ending at index:
     * <blockquote><pre>
     *     dstBegin + (srcEnd-srcBegin) - 1
     * </pre></blockquote>
     *
     * @param      srcBegin   index of the first character in the string
     *                        to copy.
     * @param      srcEnd     index after the last character in the string
     *                        to copy.
     * @param      dst        the destination array.
     * @param      dstBegin   the start offset in the destination array.
     * @exception IndexOutOfBoundsException If any of the following
     *            is true:
     *            <ul><li>{@code srcBegin} is negative.
     *            <li>{@code srcBegin} is greater than {@code srcEnd}
     *            <li>{@code srcEnd} is greater than the length of this
     *                string
     *            <li>{@code dstBegin} is negative
     *            <li>{@code dstBegin+(srcEnd-srcBegin)} is larger than
     *                {@code dst.length}</ul>
     */
    public void getChars(int srcBegin, int srcEnd, char dst[], int dstBegin) {
        if (srcBegin < 0) {
            throw new StringIndexOutOfBoundsException(srcBegin);
        }
        if (srcEnd > value.length) {
            throw new StringIndexOutOfBoundsException(srcEnd);
        }
        if (srcBegin > srcEnd) {
            throw new StringIndexOutOfBoundsException(srcEnd - srcBegin);
        }
        System.arraycopy(value, srcBegin, dst, dstBegin, srcEnd - srcBegin);
    }

 StringBuffer源码：

 public final class StringBuffer
    extends AbstractStringBuilder
    implements java.io.Serializable, CharSequence
{
 
    /**
     * A cache of the last value returned by toString. Cleared
     * whenever the StringBuffer is modified.
     */
    private transient char[] toStringCache;
 
    /** 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}
     *               argument is less than {@code 0}.
     */
    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} plus the length of the string argument.
     *
     * @param   str   the initial contents of the buffer.
     */
    public StringBuffer(String str) {
        super(str.length() + 16);
        append(str);
    }
 
    /**
     * Constructs a string buffer that contains the same characters
     * as the specified {@code CharSequence}. The initial capacity of
     * the string buffer is {@code 16} plus the length of the
     * {@code CharSequence} argument.
     * <p>
     * If the length of the specified {@code CharSequence} is
     * less than or equal to zero, then an empty buffer of capacity
     * {@code 16} is returned.
     *
     * @param      seq   the sequence to copy.
     * @since 1.5
     */
    public StringBuffer(CharSequence seq) {
        this(seq.length() + 16);
        append(seq);
    }
 
    @Override
    public synchronized int length() {
        return count;
    }
 
    @Override
    public synchronized int capacity() {
        return value.length;
    }
 
 
    @Override
    public synchronized void ensureCapacity(int minimumCapacity) {
        super.ensureCapacity(minimumCapacity);
    }
 
    /**
     * @since      1.5
     */
    @Override
    public synchronized void trimToSize() {
        super.trimToSize();
    }
 
    /**
     * @throws IndexOutOfBoundsException {@inheritDoc}
     * @see        #length()
     */
    @Override
    public synchronized void setLength(int newLength) {
        toStringCache = null;
        super.setLength(newLength);
    }
 
    /**
     * @throws IndexOutOfBoundsException {@inheritDoc}
     * @see        #length()
     */
    @Override
    public synchronized char charAt(int index) {
        if ((index < 0) || (index >= count))
            throw new StringIndexOutOfBoundsException(index);
        return value[index];
    }
 
    /**
     * @since      1.5
     */
    @Override
    public synchronized int codePointAt(int index) {
        return super.codePointAt(index);
    }
 
    /**
     * @since     1.5
     */
    @Override
    public synchronized int codePointBefore(int index) {
        return super.codePointBefore(index);
    }
 
    /**
     * @since     1.5
     */
    @Override
    public synchronized int codePointCount(int beginIndex, int endIndex) {
        return super.codePointCount(beginIndex, endIndex);
    }
 
    /**
     * @since     1.5
     */
    @Override
    public synchronized int offsetByCodePoints(int index, int codePointOffset) {
        return super.offsetByCodePoints(index, codePointOffset);
    }
 
    /**
     * @throws IndexOutOfBoundsException {@inheritDoc}
     */
    @Override
    public synchronized void getChars(int srcBegin, int srcEnd, char[] dst,
                                      int dstBegin)
    {
        super.getChars(srcBegin, srcEnd, dst, dstBegin);
    }
 
    /**
     * @throws IndexOutOfBoundsException {@inheritDoc}
     * @see        #length()
     */
    @Override
    public synchronized void setCharAt(int index, char ch) {
        if ((index < 0) || (index >= count))
            throw new StringIndexOutOfBoundsException(index);
        toStringCache = null;
        value[index] = ch;
    }
 
    @Override
    public synchronized StringBuffer append(Object obj) {
        toStringCache = null;
        super.append(String.valueOf(obj));
        return this;
    }
 
    @Override
    public synchronized StringBuffer append(String str) {
        toStringCache = null;
        super.append(str);
        return this;
    }
 
    /**
     * Appends the specified {@code StringBuffer} to this sequence.
     * <p>
     * The characters of the {@code StringBuffer} argument are appended,
     * in order, to the contents of this {@code StringBuffer}, increasing the
     * length of this {@code StringBuffer} by the length of the argument.
     * If {@code sb} is {@code null}, then the four characters
     * {@code "null"} are appended to this {@code StringBuffer}.
     * <p>
     * Let <i>n</i> be the length of the old character sequence, the one
     * contained in the {@code StringBuffer} just prior to execution of the
     * {@code append} method. Then the character at index <i>k</i> in
     * the new character sequence is equal to the character at index <i>k</i>
     * in the old character sequence, if <i>k</i> is less than <i>n</i>;
     * otherwise, it is equal to the character at index <i>k-n</i> in the
     * argument {@code sb}.
     * <p>
     * This method synchronizes on {@code this}, the destination
     * object, but does not synchronize on the source ({@code sb}).
     *
     * @param   sb   the {@code StringBuffer} to append.
     * @return  a reference to this object.
     * @since 1.4
     */
    public synchronized StringBuffer append(StringBuffer sb) {
        toStringCache = null;
        super.append(sb);
        return this;
    }
 
    /**
     * @since 1.8
     */
    @Override
    synchronized StringBuffer append(AbstractStringBuilder asb) {
        toStringCache = null;
        super.append(asb);
        return this;
    }
 
    /**
     * Appends the specified {@code CharSequence} to this
     * sequence.
     * <p>
     * The characters of the {@code CharSequence} argument are appended,
     * in order, increasing the length of this sequence by the length of the
     * argument.
     *
     * <p>The result of this method is exactly the same as if it were an
     * invocation of this.append(s, 0, s.length());
     *
     * <p>This method synchronizes on {@code this}, the destination
     * object, but does not synchronize on the source ({@code s}).
     *
     * <p>If {@code s} is {@code null}, then the four characters
     * {@code "null"} are appended.
     *
     * @param   s the {@code CharSequence} to append.
     * @return  a reference to this object.
     * @since 1.5
     */
    @Override
    public synchronized StringBuffer append(CharSequence s) {
        toStringCache = null;
        super.append(s);
        return this;
    }
 
    /**
     * @throws IndexOutOfBoundsException {@inheritDoc}
     * @since      1.5
     */
    @Override
    public synchronized StringBuffer append(CharSequence s, int start, int end)
    {
        toStringCache = null;
        super.append(s, start, end);
        return this;
    }
 
    @Override
    public synchronized StringBuffer append(char[] str) {
        toStringCache = null;
        super.append(str);
        return this;
    }
 
    /**
     * @throws IndexOutOfBoundsException {@inheritDoc}
     */
    @Override
    public synchronized StringBuffer append(char[] str, int offset, int len) {
        toStringCache = null;
        super.append(str, offset, len);
        return this;
    }
 
    @Override
    public synchronized StringBuffer append(boolean b) {
        toStringCache = null;
        super.append(b);
        return this;
    }
 
    @Override
    public synchronized StringBuffer append(char c) {
        toStringCache = null;
        super.append(c);
        return this;
    }
 
    @Override
    public synchronized StringBuffer append(int i) {
        toStringCache = null;
        super.append(i);
        return this;
    }
 
    /**
     * @since 1.5
     */
    @Override
    public synchronized StringBuffer appendCodePoint(int codePoint) {
        toStringCache = null;
        super.appendCodePoint(codePoint);
        return this;
    }
 
    @Override
    public synchronized StringBuffer append(long lng) {
        toStringCache = null;
        super.append(lng);
        return this;
    }
 
    @Override
    public synchronized StringBuffer append(float f) {
        toStringCache = null;
        super.append(f);
        return this;
    }
 
    @Override
    public synchronized StringBuffer append(double d) {
        toStringCache = null;
        super.append(d);
        return this;
    }
 
    /**
     * @throws StringIndexOutOfBoundsException {@inheritDoc}
     * @since      1.2
     */
    @Override
    public synchronized StringBuffer delete(int start, int end) {
        toStringCache = null;
        super.delete(start, end);
        return this;
    }
 
    /**
     * @throws StringIndexOutOfBoundsException {@inheritDoc}
     * @since      1.2
     */
    @Override
    public synchronized StringBuffer deleteCharAt(int index) {
        toStringCache = null;
        super.deleteCharAt(index);
        return this;
    }
 
    /**
     * @throws StringIndexOutOfBoundsException {@inheritDoc}
     * @since      1.2
     */
    @Override
    public synchronized StringBuffer replace(int start, int end, String str) {
        toStringCache = null;
        super.replace(start, end, str);
        return this;
    }
 
    /**
     * @throws StringIndexOutOfBoundsException {@inheritDoc}
     * @since      1.2
     */
    @Override
    public synchronized String substring(int start) {
        return substring(start, count);
    }
 
    /**
     * @throws IndexOutOfBoundsException {@inheritDoc}
     * @since      1.4
     */
    @Override
    public synchronized CharSequence subSequence(int start, int end) {
        return super.substring(start, end);
    }
 
    /**
     * @throws StringIndexOutOfBoundsException {@inheritDoc}
     * @since      1.2
     */
    @Override
    public synchronized String substring(int start, int end) {
        return super.substring(start, end);
    }
 
    /**
     * @throws StringIndexOutOfBoundsException {@inheritDoc}
     * @since      1.2
     */
    @Override
    public synchronized StringBuffer insert(int index, char[] str, int offset,
                                            int len)
    {
        toStringCache = null;
        super.insert(index, str, offset, len);
        return this;
    }
 
    /**
     * @throws StringIndexOutOfBoundsException {@inheritDoc}
     */
    @Override
    public synchronized StringBuffer insert(int offset, Object obj) {
        toStringCache = null;
        super.insert(offset, String.valueOf(obj));
        return this;
    }
 
    /**
     * @throws StringIndexOutOfBoundsException {@inheritDoc}
     */
    @Override
    public synchronized StringBuffer insert(int offset, String str) {
        toStringCache = null;
        super.insert(offset, str);
        return this;
    }
 
    /**
     * @throws StringIndexOutOfBoundsException {@inheritDoc}
     */
    @Override
    public synchronized StringBuffer insert(int offset, char[] str) {
        toStringCache = null;
        super.insert(offset, str);
        return this;
    }
 
    /**
     * @throws IndexOutOfBoundsException {@inheritDoc}
     * @since      1.5
     */
    @Override
    public StringBuffer insert(int dstOffset, CharSequence s) {
        // Note, synchronization achieved via invocations of other StringBuffer methods
        // after narrowing of s to specific type
        // Ditto for toStringCache clearing
        super.insert(dstOffset, s);
        return this;
    }
 
    /**
     * @throws IndexOutOfBoundsException {@inheritDoc}
     * @since      1.5
     */
    @Override
    public synchronized StringBuffer insert(int dstOffset, CharSequence s,
            int start, int end)
    {
        toStringCache = null;
        super.insert(dstOffset, s, start, end);
        return this;
    }
 
    /**
     * @throws StringIndexOutOfBoundsException {@inheritDoc}
     */
    @Override
    public  StringBuffer insert(int offset, boolean b) {
        // Note, synchronization achieved via invocation of StringBuffer insert(int, String)
        // after conversion of b to String by super class method
        // Ditto for toStringCache clearing
        super.insert(offset, b);
        return this;
    }
 
    /**
     * @throws IndexOutOfBoundsException {@inheritDoc}
     */
    @Override
    public synchronized StringBuffer insert(int offset, char c) {
        toStringCache = null;
        super.insert(offset, c);
        return this;
    }
 
    /**
     * @throws StringIndexOutOfBoundsException {@inheritDoc}
     */
    @Override
    public StringBuffer insert(int offset, int i) {
        // Note, synchronization achieved via invocation of StringBuffer insert(int, String)
        // after conversion of i to String by super class method
        // Ditto for toStringCache clearing
        super.insert(offset, i);
        return this;
    }
 
    /**
     * @throws StringIndexOutOfBoundsException {@inheritDoc}
     */
    @Override
    public StringBuffer insert(int offset, long l) {
        // Note, synchronization achieved via invocation of StringBuffer insert(int, String)
        // after conversion of l to String by super class method
        // Ditto for toStringCache clearing
        super.insert(offset, l);
        return this;
    }
 
    /**
     * @throws StringIndexOutOfBoundsException {@inheritDoc}
     */
    @Override
    public StringBuffer insert(int offset, float f) {
        // Note, synchronization achieved via invocation of StringBuffer insert(int, String)
        // after conversion of f to String by super class method
        // Ditto for toStringCache clearing
        super.insert(offset, f);
        return this;
    }
 
    /**
     * @throws StringIndexOutOfBoundsException {@inheritDoc}
     */
    @Override
    public StringBuffer insert(int offset, double d) {
        // Note, synchronization achieved via invocation of StringBuffer insert(int, String)
        // after conversion of d to String by super class method
        // Ditto for toStringCache clearing
        super.insert(offset, d);
        return this;
    }
 
    /**
     * @since      1.4
     */
    @Override
    public int indexOf(String str) {
        // Note, synchronization achieved via invocations of other StringBuffer methods
        return super.indexOf(str);
    }
 
    /**
     * @since      1.4
     */
    @Override
    public synchronized int indexOf(String str, int fromIndex) {
        return super.indexOf(str, fromIndex);
    }
 
    /**
     * @since      1.4
     */
    @Override
    public int lastIndexOf(String str) {
        // Note, synchronization achieved via invocations of other StringBuffer methods
        return lastIndexOf(str, count);
    }
 
    /**
     * @since      1.4
     */
    @Override
    public synchronized int lastIndexOf(String str, int fromIndex) {
        return super.lastIndexOf(str, fromIndex);
    }
 
    /**
     * @since   JDK1.0.2
     */
    @Override
    public synchronized StringBuffer reverse() {
        toStringCache = null;
        super.reverse();
        return this;
    }
 
    @Override
    public synchronized String toString() {
        if (toStringCache == null) {
            toStringCache = Arrays.copyOfRange(value, 0, count);
        }
        return new String(toStringCache, true);
    }
 
    /**
     * Serializable fields for StringBuffer.
     *
     * @serialField value  char[]
     *              The backing character array of this StringBuffer.
     * @serialField count int
     *              The number of characters in this StringBuffer.
     * @serialField shared  boolean
     *              A flag indicating whether the backing array is shared.
     *              The value is ignored upon deserialization.
     */
    private static final java.io.ObjectStreamField[] serialPersistentFields =
    {
        new java.io.ObjectStreamField("value", char[].class),
        new java.io.ObjectStreamField("count", Integer.TYPE),
        new java.io.ObjectStreamField("shared", Boolean.TYPE),
    };
 
    /**
     * readObject is called to restore the state of the StringBuffer from
     * a stream.
     */
    private synchronized void writeObject(java.io.ObjectOutputStream s)
        throws java.io.IOException {
        java.io.ObjectOutputStream.PutField fields = s.putFields();
        fields.put("value", value);
        fields.put("count", count);
        fields.put("shared", false);
        s.writeFields();
    }
 
    /**
     * readObject is called to restore the state of the StringBuffer from
     * a stream.
     */
    private void readObject(java.io.ObjectInputStream s)
        throws java.io.IOException, ClassNotFoundException {
        java.io.ObjectInputStream.GetField fields = s.readFields();
        value = (char[])fields.get("value", null);
        count = fields.get("count", 0);
    }
}

StringBuilder

StringBuilder的实现与StringBuffer代码上差不多，但StringBuffer是使用了锁是线程安全的。

public final class StringBuilder
    extends AbstractStringBuilder
    implements java.io.Serializable, CharSequence
{
 
    /** use serialVersionUID for interoperability */
    static final long serialVersionUID = 4383685877147921099L;
 
    /**
     * 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} argument.
     *
     * @param      capacity  the initial capacity.
     * @throws     NegativeArraySizeException  if the {@code capacity}
     *               argument is less than {@code 0}.
     */
    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} plus the length of the string argument.
     *
     * @param   str   the initial contents of the buffer.
     */
    public StringBuilder(String str) {
        super(str.length() + 16);
        append(str);
    }
 
    /**
     * Constructs a string builder that contains the same characters
     * as the specified {@code CharSequence}. The initial capacity of
     * the string builder is {@code 16} plus the length of the
     * {@code CharSequence} argument.
     *
     * @param      seq   the sequence to copy.
     */
    public StringBuilder(CharSequence seq) {
        this(seq.length() + 16);
        append(seq);
    }
 
    @Override
    public StringBuilder append(Object obj) {
        return append(String.valueOf(obj));
    }
 
    @Override
    public StringBuilder append(String str) {
        super.append(str);
        return this;
    }
 
    /**
     * Appends the specified {@code StringBuffer} to this sequence.
     * <p>
     * The characters of the {@code StringBuffer} argument are appended,
     * in order, to this sequence, increasing the
     * length of this sequence by the length of the argument.
     * If {@code sb} is {@code null}, then the four characters
     * {@code "null"} are appended to this sequence.
     * <p>
     * Let <i>n</i> be the length of this character sequence just prior to
     * execution of the {@code append} method. Then the character at index
     * <i>k</i> in the new character sequence is equal to the character at
     * index <i>k</i> in the old character sequence, if <i>k</i> is less than
     * <i>n</i>; otherwise, it is equal to the character at index <i>k-n</i>
     * in the argument {@code sb}.
     *
     * @param   sb   the {@code StringBuffer} to append.
     * @return  a reference to this object.
     */
    public StringBuilder append(StringBuffer sb) {
        super.append(sb);
        return this;
    }
 
    @Override
    public StringBuilder append(CharSequence s) {
        super.append(s);
        return this;
    }
 
    /**
     * @throws     IndexOutOfBoundsException {@inheritDoc}
     */
    @Override
    public StringBuilder append(CharSequence s, int start, int end) {
        super.append(s, start, end);
        return this;
    }
 
    @Override
    public StringBuilder append(char[] str) {
        super.append(str);
        return this;
    }
 
    /**
     * @throws IndexOutOfBoundsException {@inheritDoc}
     */
    @Override
    public StringBuilder append(char[] str, int offset, int len) {
        super.append(str, offset, len);
        return this;
    }
 
    @Override
    public StringBuilder append(boolean b) {
        super.append(b);
        return this;
    }
 
    @Override
    public StringBuilder append(char c) {
        super.append(c);
        return this;
    }
 
    @Override
    public StringBuilder append(int i) {
        super.append(i);
        return this;
    }
 
    @Override
    public StringBuilder append(long lng) {
        super.append(lng);
        return this;
    }
 
    @Override
    public StringBuilder append(float f) {
        super.append(f);
        return this;
    }
 
    @Override
    public StringBuilder append(double d) {
        super.append(d);
        return this;
    }
 
    /**
     * @since 1.5
     */
    @Override
    public StringBuilder appendCodePoint(int codePoint) {
        super.appendCodePoint(codePoint);
        return this;
    }
 
    /**
     * @throws StringIndexOutOfBoundsException {@inheritDoc}
     */
    @Override
    public StringBuilder delete(int start, int end) {
        super.delete(start, end);
        return this;
    }
 
    /**
     * @throws StringIndexOutOfBoundsException {@inheritDoc}
     */
    @Override
    public StringBuilder deleteCharAt(int index) {
        super.deleteCharAt(index);
        return this;
    }
 
    /**
     * @throws StringIndexOutOfBoundsException {@inheritDoc}
     */
    @Override
    public StringBuilder replace(int start, int end, String str) {
        super.replace(start, end, str);
        return this;
    }
 
    /**
     * @throws StringIndexOutOfBoundsException {@inheritDoc}
     */
    @Override
    public StringBuilder insert(int index, char[] str, int offset,
                                int len)
    {
        super.insert(index, str, offset, len);
        return this;
    }
 
    /**
     * @throws StringIndexOutOfBoundsException {@inheritDoc}
     */
    @Override
    public StringBuilder insert(int offset, Object obj) {
            super.insert(offset, obj);
            return this;
    }
 
    /**
     * @throws StringIndexOutOfBoundsException {@inheritDoc}
     */
    @Override
    public StringBuilder insert(int offset, String str) {
        super.insert(offset, str);
        return this;
    }
 
    /**
     * @throws StringIndexOutOfBoundsException {@inheritDoc}
     */
    @Override
    public StringBuilder insert(int offset, char[] str) {
        super.insert(offset, str);
        return this;
    }
 
    /**
     * @throws IndexOutOfBoundsException {@inheritDoc}
     */
    @Override
    public StringBuilder insert(int dstOffset, CharSequence s) {
            super.insert(dstOffset, s);
            return this;
    }
 
    /**
     * @throws IndexOutOfBoundsException {@inheritDoc}
     */
    @Override
    public StringBuilder insert(int dstOffset, CharSequence s,
                                int start, int end)
    {
        super.insert(dstOffset, s, start, end);
        return this;
    }
 
    /**
     * @throws StringIndexOutOfBoundsException {@inheritDoc}
     */
    @Override
    public StringBuilder insert(int offset, boolean b) {
        super.insert(offset, b);
        return this;
    }
 
    /**
     * @throws IndexOutOfBoundsException {@inheritDoc}
     */
    @Override
    public StringBuilder insert(int offset, char c) {
        super.insert(offset, c);
        return this;
    }
 
    /**
     * @throws StringIndexOutOfBoundsException {@inheritDoc}
     */
    @Override
    public StringBuilder insert(int offset, int i) {
        super.insert(offset, i);
        return this;
    }
 
    /**
     * @throws StringIndexOutOfBoundsException {@inheritDoc}
     */
    @Override
    public StringBuilder insert(int offset, long l) {
        super.insert(offset, l);
        return this;
    }
 
    /**
     * @throws StringIndexOutOfBoundsException {@inheritDoc}
     */
    @Override
    public StringBuilder insert(int offset, float f) {
        super.insert(offset, f);
        return this;
    }
 
    /**
     * @throws StringIndexOutOfBoundsException {@inheritDoc}
     */
    @Override
    public StringBuilder insert(int offset, double d) {
        super.insert(offset, d);
        return this;
    }
 
    @Override
    public int indexOf(String str) {
        return super.indexOf(str);
    }
 
    @Override
    public int indexOf(String str, int fromIndex) {
        return super.indexOf(str, fromIndex);
    }
 
    @Override
    public int lastIndexOf(String str) {
        return super.lastIndexOf(str);
    }
 
    @Override
    public int lastIndexOf(String str, int fromIndex) {
        return super.lastIndexOf(str, fromIndex);
    }
 
    @Override
    public StringBuilder reverse() {
        super.reverse();
        return this;
    }
 
    @Override
    public String toString() {
        // Create a copy, don't share the array
        return new String(value, 0, count);
    }
 
    /**
     * Save the state of the {@code StringBuilder} instance to a stream
     * (that is, serialize it).
     *
     * @serialData the number of characters currently stored in the string
     *             builder ({@code int}), followed by the characters in the
     *             string builder ({@code char[]}).   The length of the
     *             {@code char} array may be greater than the number of
     *             characters currently stored in the string builder, in which
     *             case extra characters are ignored.
     */
    private void writeObject(java.io.ObjectOutputStream s)
        throws java.io.IOException {
        s.defaultWriteObject();
        s.writeInt(count);
        s.writeObject(value);
    }
 
    /**
     * readObject is called to restore the state of the StringBuffer from
     * a stream.
     */
    private void readObject(java.io.ObjectInputStream s)
        throws java.io.IOException, ClassNotFoundException {
        s.defaultReadObject();
        count = s.readInt();
        value = (char[]) s.readObject();
    }
 
}

如何推导结论？

众所周知，static 的变量是线程之间共享的，加synchronized字段的方法和代码块在多线程调度中也能起到资源集中占用的优势。

结论：线程安全的就是独占的，所以效率上StringBuffer比StringBuilder的效率要高。