String / StringBuilder / StringBuffer

1. String 不可变的原因

直接上源码（摘自 JDK 11.0.10 ）— 仅取部分源码非全部

public final class String implements java.io.Serializable, Comparable<String>, CharSequence {
    @Stable
    private final byte[] value;
}

从源码中可以看到 String 类中使用 final 修饰一个 byte[] 来保存字符串，因此 String 对象是不可变的。

2. StringBuilder

源码（摘自 JDK 11.0.10）— 仅取部分源码非全部

public final class StringBuilder
    extends AbstractStringBuilder
    implements java.io.Serializable, Comparable<StringBuilder>, CharSequence
{

    /**
     * Constructs a string builder with no characters in it and an
     * initial capacity of 16 characters.
     */
    @HotSpotIntrinsicCandidate
    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}.
     */
    @HotSpotIntrinsicCandidate
    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.
     */
    @HotSpotIntrinsicCandidate
    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);
    }
  
}

从上述源码中可以看到 StringBuilder 继承于 AbstractStringBuilder 类，所以继续跟踪，查看 AbstractStringBuilder 类的源码（仅取部分源码非全部）如下：

abstract class AbstractStringBuilder implements Appendable, CharSequence {
    /**
     * The value is used for character storage.
     */
    byte[] value;

    /**
     * The id of the encoding used to encode the bytes in {@code value}.
     */
    byte coder;

    /**
     * The count is the number of characters used.
     */
    int count;

    private static final byte[] EMPTYVALUE = new byte[0];

    /**
     * This no-arg constructor is necessary for serialization of subclasses.
     */
    AbstractStringBuilder() {
        value = EMPTYVALUE;
    }

    /**
     * Creates an AbstractStringBuilder of the specified capacity.
     */
    AbstractStringBuilder(int capacity) {
        if (COMPACT_STRINGS) {
            value = new byte[capacity];
            coder = LATIN1;
        } else {
            value = StringUTF16.newBytesFor(capacity);
            coder = UTF16;
        }
    }
}

从 AbstractStringBuilder 类中可以发现，其使用 byte[] 来存储字符串（没有被 final 修饰），所以 StringBuilder 的对象是可变的。从 StringBuilder 的源码中也可以看出，StringBuilder 的构造方法实际上使用的是父类 AbstractStringBuilder 的构造方法，并且可以从中看出，默认 StringBuilder 对象的大小为 16，如果给定初始化对象，则初始大小为：初始化对象长度 + 16。

3. StringBuffer

源码（摘自 JDK 11.0.10）— 仅取部分源码非全部

public final class StringBuffer
    extends AbstractStringBuilder
    implements java.io.Serializable, Comparable<StringBuffer>, CharSequence
{

    /**
     * A cache of the last value returned by toString. Cleared
     * whenever the StringBuffer is modified.
     */
    private transient String 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.
     */
    @HotSpotIntrinsicCandidate
    public StringBuffer() {
        super(16);
    }

    /**
     * Constructs a string buffer with no characters in it and
     * the specified initial capacity.
     *
     * @param      capacity  the initial capacity.
     * @throws     NegativeArraySizeException  if the {@code capacity}
     *             argument is less than {@code 0}.
     */
    @HotSpotIntrinsicCandidate
    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.
     */
    @HotSpotIntrinsicCandidate
    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 compareTo(StringBuffer another) {
        return super.compareTo(another);
    }

    @Override
    public synchronized int length() {
        return count;
    }

    @Override
    public synchronized int capacity() {
        return super.capacity();
    }


    @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) {
        return super.charAt(index);
    }

    /**
     * @throws IndexOutOfBoundsException {@inheritDoc}
     * @since      1.5
     */
    @Override
    public synchronized int codePointAt(int index) {
        return super.codePointAt(index);
    }

    /**
     * @throws IndexOutOfBoundsException {@inheritDoc}
     * @since     1.5
     */
    @Override
    public synchronized int codePointBefore(int index) {
        return super.codePointBefore(index);
    }

    /**
     * @throws IndexOutOfBoundsException {@inheritDoc}
     * @since     1.5
     */
    @Override
    public synchronized int codePointCount(int beginIndex, int endIndex) {
        return super.codePointCount(beginIndex, endIndex);
    }

    /**
     * @throws IndexOutOfBoundsException {@inheritDoc}
     * @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) {
        toStringCache = null;
        super.setCharAt(index, ch);
    }
}

从源码中可以看到 StringBuffer 同样继承于 AbstractStringBuilder 类。所以在对象可变性上，StringBuffer 与 StringBuilder 相同，在对 byte[] 进行初始化时和 StringBuilder 相同。即默认对象的大小为 16，当给定初始化对象时，则初始大小为：初始化对象长度 + 16。
从 StringBuffer 的方法中可以看到所有的方法加了 synchronized 关键字——同步锁，所以 StringBuffer 是线程安全的。