stringBuilder 源码学习

jdk: 1.8

类定义

/**
 * A mutable sequence of characters.  This class provides an API compatible
 * with {@code StringBuffer}, but with no guarantee of synchronization.
 * This class is designed for use as a drop-in replacement for
 * {@code StringBuffer} in places where the string buffer was being
 * used by a single thread (as is generally the case).   Where possible,
 * it is recommended that this class be used in preference to
 * {@code StringBuffer} as it will be faster under most implementations.
 *
 * <p>The principal operations on a {@code StringBuilder} are the
 * {@code append} and {@code insert} 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 builder. The
 * {@code append} method always adds these characters at the end
 * of the builder; the {@code insert} method adds the characters at
 * a specified point.
 * <p>
 * For example, if {@code z} refers to a string builder object
 * whose current contents are "{@code start}", then
 * the method call {@code z.append("le")} would cause the string
 * builder to contain "{@code startle}", whereas
 * {@code z.insert(4, "le")} would alter the string builder to
 * contain "{@code starlet}".
 * <p>
 * In general, if sb refers to an instance of a {@code StringBuilder},
 * then {@code sb.append(x)} has the same effect as
 * {@code sb.insert(sb.length(), x)}.
 * <p>
 * Every string builder has a capacity. As long as the length of the
 * character sequence contained in the string builder does not exceed
 * the capacity, it is not necessary to allocate a new internal
 * buffer. If the internal buffer overflows, it is automatically made larger.
 *
 * <p>Instances of {@code StringBuilder} are not safe for
 * use by multiple threads. If such synchronization is required then it is
 * recommended that {@link java.lang.StringBuffer} be used.
 *
 * <p>Unless otherwise noted, passing a {@code null} argument to a constructor
 * or method in this class will cause a {@link NullPointerException} to be
 * thrown.
 *
 * @author      Michael McCloskey
 * @see         java.lang.StringBuffer
 * @see         java.lang.String
 * @since       1.5
 */
public final class StringBuilder
    extends AbstractStringBuilder
    implements Serializable, CharSequence
{

    /** use serialVersionUID for interoperability */
    static final long serialVersionUID = 4383685877147921099L;

接口

实现了 Serializable 和 CharSequence 接口。

CharSequence 接口

public interface CharSequence {

    /**
     * 长度
     */
    int length();

    /**
     * 指定位置
     */
    char charAt(int index);

    /**
     * 截取
     */
    CharSequence subSequence(int start, int end);

    /**
     * Returns a string containing the characters in this sequence in the same
     * order as this sequence.  The length of the string will be the length of
     * this sequence.
     *
     * @return  a string consisting of exactly this sequence of characters
     */
    public String toString();

    /**
     * Returns a stream of {@code int} zero-extending the {@code char} values
     * from this sequence.  Any char which maps to a <a
     * href="{@docRoot}/java/lang/Character.html#unicode">surrogate code
     * point</a> is passed through uninterpreted.
     *
     * <p>If the sequence is mutated while the stream is being read, the
     * result is undefined.
     *
     * @return an IntStream of char values from this sequence
     * @since 1.8
     */
    public default IntStream chars() {
        // 迭代器
        class CharIterator implements PrimitiveIterator.OfInt {
            int cur = 0;

            public boolean hasNext() {
                return cur < length();
            }

            public int nextInt() {
                if (hasNext()) {
                    return charAt(cur++);
                } else {
                    throw new NoSuchElementException();
                }
            }

            @Override
            public void forEachRemaining(IntConsumer block) {
                for (; cur < length(); cur++) {
                    block.accept(charAt(cur));
                }
            }
        }

        return StreamSupport.intStream(() ->
                Spliterators.spliterator(
                        new CharIterator(),
                        length(),
                        Spliterator.ORDERED),
                Spliterator.SUBSIZED | Spliterator.SIZED | Spliterator.ORDERED,
                false);
    }

    /**
     * Returns a stream of code point values from this sequence.  Any surrogate
     * pairs encountered in the sequence are combined as if by {@linkplain
     * Character#toCodePoint Character.toCodePoint} and the result is passed
     * to the stream. Any other code units, including ordinary BMP characters,
     * unpaired surrogates, and undefined code units, are zero-extended to
     * {@code int} values which are then passed to the stream.
     *
     * <p>If the sequence is mutated while the stream is being read, the result
     * is undefined.
     *
     * @return an IntStream of Unicode code points from this sequence
     * @since 1.8
     */
    public default IntStream codePoints() {
        class CodePointIterator implements PrimitiveIterator.OfInt {
            int cur = 0;

            @Override
            public void forEachRemaining(IntConsumer block) {
                final int length = length();
                int i = cur;
                try {
                    while (i < length) {
                        char c1 = charAt(i++);
                        if (!Character.isHighSurrogate(c1) || i >= length) {
                            block.accept(c1);
                        } else {
                            char c2 = charAt(i);
                            if (Character.isLowSurrogate(c2)) {
                                i++;
                                block.accept(Character.toCodePoint(c1, c2));
                            } else {
                                block.accept(c1);
                            }
                        }
                    }
                } finally {
                    cur = i;
                }
            }

            public boolean hasNext() {
                return cur < length();
            }

            public int nextInt() {
                final int length = length();

                if (cur >= length) {
                    throw new NoSuchElementException();
                }
                char c1 = charAt(cur++);
                if (Character.isHighSurrogate(c1) && cur < length) {
                    char c2 = charAt(cur);
                    if (Character.isLowSurrogate(c2)) {
                        cur++;
                        return Character.toCodePoint(c1, c2);
                    }
                }
                return c1;
            }
        }

        return StreamSupport.intStream(() ->
                Spliterators.spliteratorUnknownSize(
                        new CodePointIterator(),
                        Spliterator.ORDERED),
                Spliterator.ORDERED,
                false);
    }
}

构造器

    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);
    }

char[]

这里直接继承了父类的构造器：

    AbstractStringBuilder() {
    }

    /**
     * Creates an AbstractStringBuilder of the specified capacity.
     */
    AbstractStringBuilder(int capacity) {
        value = new char[capacity];
    }

创建一个指定大小的 char 数组

各种 append 方法

append(string)

还有就是初始化对应的 seq 序列，通过 append 方法：

    @Override
    public StringBuilder append(String str) {
        super.append(str);
        return this;
    }

引用父类：

    public AbstractStringBuilder append(String str) {
        if (str == null)
            // null 直接添加 'null'
            return appendNull();
        int len = str.length();
        ensureCapacityInternal(count + len);
        str.getChars(0, len, value, count);
        count += len;
        return this;
    }

可能需要扩容：

    private void ensureCapacityInternal(int minimumCapacity) {
        // overflow-conscious code
        if (minimumCapacity - value.length > 0) {
            // 通过数组 copy 的方式
            value = Arrays.copyOf(value,
                    newCapacity(minimumCapacity));
        }
    }

新容量的计算：

    private int newCapacity(int minCapacity) {
        // overflow-conscious code
        // 翻倍+2
        int newCapacity = (value.length << 1) + 2;
        if (newCapacity - minCapacity < 0) {
            newCapacity = minCapacity;
        }
        return (newCapacity <= 0 || MAX_ARRAY_SIZE - newCapacity < 0)
            ? hugeCapacity(minCapacity)
            : newCapacity;
    }

其中的 getChars 实现如下：

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);
    }

    // 系统的数组拷贝，性能比较好。
    System.arraycopy(value, srcBegin, dst, dstBegin, srcEnd - srcBegin);
}

append(chars[], start, end)

    @Override
    public StringBuilder append(char[] str, int offset, int len) {
        super.append(str, offset, len);
        return this;
    }

底层实现依然是通过系统的数组拷贝：

public AbstractStringBuilder append(char str[], int offset, int len) {
    if (len > 0)                // let arraycopy report AIOOBE for len < 0
        ensureCapacityInternal(count + len);
    System.arraycopy(str, offset, value, count, len);
    count += len;
    return this;
}

当然依然涉及到一次可能的扩容。

toString()

public String toString() {
    // Create a copy, don't share the array
    return new String(value, 0, count);
}

这里创建了一个 copy，新的 String。

参考资料