JDK源码阅读--StringBuffer

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;//被transient修饰，表示这个字段不被序列化

StringBuffer是被final修饰的，表示不能被其它类继承，；继承了AbstractStringBuilder抽象类，实现了Serializable、CharSequence接口

StringBuffer中的方法被public修饰的都使用了synchronized，保证线程安全。所以StringBuffer是线程安全的类。

/**
     * 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);
    }

StringBuffer的容量默认是16，当容量不够的时候，容量会继续增加16，依次类推。