StringBuffer

类图：

image.png

类：

 public final class StringBuffer
    extends AbstractStringBuilder
    implements java.io.Serializable, CharSequence
{}

构造函数：

    public StringBuffer() {
        super(16);
    }


    public StringBuffer(int capacity) {
        super(capacity);
    }


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

 
    public StringBuffer(CharSequence seq) {
        this(seq.length() + 16);
        append(seq);
    }

append 方法带 synchronized 同步锁 （同时只有一个线程执行，所以也是线程安全的）

    @Override
    public synchronized StringBuffer append(String str) {
        toStringCache = null;
        super.append(str);
        return this;
    }


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