关于StringBuffer.setLength和trimToSize

首先声明jdk-version:7u40

好了,先新建一个StringBuffer.

StringBuffer aa = new StringBuffer("12345");

下面是jdk源码:

public StringBuffer(String str) {
        super(str.length() + 16);
        append(str);
}

super() :

AbstractStringBuilder(int capacity) {
        value = new char[capacity];
}

也就是说,StringBuffer新建了容量为字符串长度+16的字符数组.

StringBuffer aa = new StringBuffer("12345");
System.out.println(aa.capacity());//21
System.out.println(aa.length());//5

下面我们调用`setLength(int newLength)`

aa.setLength(16);

再次输出

System.out.println(aa.capacity());//21
System.out.println(aa.length());//16

其中16-5多出来的被`\0`补全了.

输出:

System.out.println(aa.toString());

public void setLength(int newLength) {
        if (newLength < 0)
            throw new StringIndexOutOfBoundsException(newLength);
        ensureCapacityInternal(newLength);

        if (count < newLength) {
            for (; count < newLength; count++)
                value[count] = '\0';
        } else {
            count = newLength;
        }
}

下面这句确认是否新设置的长度,大于现有char[]的长度,大于就要扩展容量.

 ensureCapacityInternal(newLength);

    public void ensureCapacity(int minimumCapacity) {
        if (minimumCapacity > 0)
            ensureCapacityInternal(minimumCapacity);
    }

    /**
     * This method has the same contract as ensureCapacity, but is
     * never synchronized.
     */
    private void ensureCapacityInternal(int minimumCapacity) {
        // overflow-conscious code
        if (minimumCapacity - value.length > 0)
            expandCapacity(minimumCapacity);
    }

    /**
     * This implements the expansion semantics of ensureCapacity with no
     * size check or synchronization.
     */
    void expandCapacity(int minimumCapacity) {
        int newCapacity = value.length * 2 + 2;
        if (newCapacity - minimumCapacity < 0)
            newCapacity = minimumCapacity;
        if (newCapacity < 0) {
            if (minimumCapacity < 0) // overflow
                throw new OutOfMemoryError();
            newCapacity = Integer.MAX_VALUE;
        }
        value = Arrays.copyOf(value, newCapacity);
    }

至于扩容...2倍+2

如果你设置的长度,小于现有的数据长度,也就是

aa.setLength(2);

那么输出:

System.out.println(aa.capacity());//21
System.out.println(aa.length());//2
System.out.println(aa.toString());//12

容量没变,记录char[]长度的成员变量值变了,至于value[]里的值并没有消失.证据是:

我调用了:

aa.append("aa");

然后打断点...

得到下面两张图:

由此可见,不仅没消失,append只是复写了它,也就是如果可以得到count的修改权,我们能找回以前的数据...不过,反正我没得到,你办到了记得通知我.

我们继续说trimToSize,解释是它会将value[](也就是char[])的长度(容量)改成数据的长度

调用:

		StringBuffer aa = new StringBuffer("12345");
		aa.setLength(2);
		aa.append("aa");
		aa.trimToSize();
		System.out.println(aa.capacity());//4
		System.out.println(aa.length());//4

源码:

/**
     * Attempts to reduce storage used for the character sequence.
     * If the buffer is larger than necessary to hold its current sequence of
     * characters, then it may be resized to become more space efficient.
     * Calling this method may, but is not required to, affect the value
     * returned by a subsequent call to the {@link #capacity()} method.
     */
    public void trimToSize() {
        if (count < value.length) {
            value = Arrays.copyOf(value, count);
        }
    }

copyOf:

/**
     * Copies the specified array, truncating or padding with null characters (if necessary)
     * so the copy has the specified length.  For all indices that are valid
     * in both the original array and the copy, the two arrays will contain
     * identical values.  For any indices that are valid in the copy but not
     * the original, the copy will contain <tt>'\\u000'</tt>.  Such indices
     * will exist if and only if the specified length is greater than that of
     * the original array.
     *
     * @param original the array to be copied
     * @param newLength the length of the copy to be returned
     * @return a copy of the original array, truncated or padded with null characters
     *     to obtain the specified length
     * @throws NegativeArraySizeException if <tt>newLength</tt> is negative
     * @throws NullPointerException if <tt>original</tt> is null
     * @since 1.6
     */
    public static char[] copyOf(char[] original, int newLength) {
        char[] copy = new char[newLength];
        System.arraycopy(original, 0, copy, 0,
                         Math.min(original.length, newLength));
        return copy;
    }

也就是说,它调用了copy数组的方法,使用现在记录的长度,复制了一个基于现数组和指定长度的数组,替代了旧数组,至于旧的,我觉得它是乖乖的去等GC了,不过没有证据,因为我追到这,就停了:

/**
     * Copies an array from the specified source array, beginning at the
     * specified position, to the specified position of the destination array.
     * A subsequence of array components are copied from the source
     * array referenced by <code>src</code> to the destination array
     * referenced by <code>dest</code>. The number of components copied is
     * equal to the <code>length</code> argument. The components at
     * positions <code>srcPos</code> through
     * <code>srcPos+length-1</code> in the source array are copied into
     * positions <code>destPos</code> through
     * <code>destPos+length-1</code>, respectively, of the destination
     * array.
     * <p>
     * If the <code>src</code> and <code>dest</code> arguments refer to the
     * same array object, then the copying is performed as if the
     * components at positions <code>srcPos</code> through
     * <code>srcPos+length-1</code> were first copied to a temporary
     * array with <code>length</code> components and then the contents of
     * the temporary array were copied into positions
     * <code>destPos</code> through <code>destPos+length-1</code> of the
     * destination array.
     * <p>
     * If <code>dest</code> is <code>null</code>, then a
     * <code>NullPointerException</code> is thrown.
     * <p>
     * If <code>src</code> is <code>null</code>, then a
     * <code>NullPointerException</code> is thrown and the destination
     * array is not modified.
     * <p>
     * Otherwise, if any of the following is true, an
     * <code>ArrayStoreException</code> is thrown and the destination is
     * not modified:
     * <ul>
     * <li>The <code>src</code> argument refers to an object that is not an
     *     array.
     * <li>The <code>dest</code> argument refers to an object that is not an
     *     array.
     * <li>The <code>src</code> argument and <code>dest</code> argument refer
     *     to arrays whose component types are different primitive types.
     * <li>The <code>src</code> argument refers to an array with a primitive
     *    component type and the <code>dest</code> argument refers to an array
     *     with a reference component type.
     * <li>The <code>src</code> argument refers to an array with a reference
     *    component type and the <code>dest</code> argument refers to an array
     *     with a primitive component type.
     * </ul>
     * <p>
     * Otherwise, if any of the following is true, an
     * <code>IndexOutOfBoundsException</code> is
     * thrown and the destination is not modified:
     * <ul>
     * <li>The <code>srcPos</code> argument is negative.
     * <li>The <code>destPos</code> argument is negative.
     * <li>The <code>length</code> argument is negative.
     * <li><code>srcPos+length</code> is greater than
     *     <code>src.length</code>, the length of the source array.
     * <li><code>destPos+length</code> is greater than
     *     <code>dest.length</code>, the length of the destination array.
     * </ul>
     * <p>
     * Otherwise, if any actual component of the source array from
     * position <code>srcPos</code> through
     * <code>srcPos+length-1</code> cannot be converted to the component
     * type of the destination array by assignment conversion, an
     * <code>ArrayStoreException</code> is thrown. In this case, let
     * <b><i>k</i></b> be the smallest nonnegative integer less than
     * length such that <code>src[srcPos+</code><i>k</i><code>]</code>
     * cannot be converted to the component type of the destination
     * array; when the exception is thrown, source array components from
     * positions <code>srcPos</code> through
     * <code>srcPos+</code><i>k</i><code>-1</code>
     * will already have been copied to destination array positions
     * <code>destPos</code> through
     * <code>destPos+</code><i>k</I><code>-1</code> and no other
     * positions of the destination array will have been modified.
     * (Because of the restrictions already itemized, this
     * paragraph effectively applies only to the situation where both
     * arrays have component types that are reference types.)
     *
     * @param      src      the source array.
     * @param      srcPos   starting position in the source array.
     * @param      dest     the destination array.
     * @param      destPos  starting position in the destination data.
     * @param      length   the number of array elements to be copied.
     * @exception  IndexOutOfBoundsException  if copying would cause
     *               access of data outside array bounds.
     * @exception  ArrayStoreException  if an element in the <code>src</code>
     *               array could not be stored into the <code>dest</code> array
     *               because of a type mismatch.
     * @exception  NullPointerException if either <code>src</code> or
     *               <code>dest</code> is <code>null</code>.
     */
    public static native void arraycopy(Object src,  int  srcPos,
                                        Object dest, int destPos,
                                        int length);

不过value的引用,指向已经替换成了copyOf回来的,那原数组也就是失去了引用,也就是不可达了吧,是吧?你也觉得它是等垃圾车了吧.

至于它是不是所谓的GC Roots对象,和GC Roots是否会被回收,我没有深究,如果你知道,请评论告诉我.

到此,就是我想记录的关于标题提到的两个方法,相关的事项.希望你能看完,虽然并没有什么卵用.