StringBuilder和StringBuffer源码浅析

首先我们来看两个类的继承体系：

public final class StringBuilder extends AbstractStringBuilder implements        Appendable, CharSequence, Serializable；public final class StringBuffer extends AbstractStringBuilder implements        Appendable, Serializable, CharSequence

二者基本类似，我们看AbstravtStringBuilder这个类

    private char[] value;    static final int INITIAL_CAPACITY = 16;    AbstractStringBuilder() {        value = new char[INITIAL_CAPACITY];    }    AbstractStringBuilder(int capacity) {        if (capacity < 0) {            throw new NegativeArraySizeException(Integer.toString(capacity));        }        value = new char[capacity];    }      AbstractStringBuilder(String string) {        count = string.length();        shared = false;        value = new char[count + INITIAL_CAPACITY];        string.getCharsNoCheck(0, count, value, 0);    }

默认初始容量为16，当然我们也可以自己设置容量大小；接着我们看看拼接过程

   final void append0(char[] chars) {        int newCount = count + chars.length;        if (newCount > value.length) {            enlargeBuffer(newCount);        }        System.arraycopy(chars, 0, value, count, chars.length);        count = newCount;    }

接下来我们看看扩容过程，

     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);    }

尝试将新容量扩为大小变成2倍+2 ，容量如果不够，直接扩充到需要的容量大小。所以我们在创建SB的时候，可以估算新字符串的长度来适当避免多次扩容，影响效率。

接着我们来看看StringBuffer的append方法

 public synchronized StringBuffer append(char[] chars) {        append0(chars);        return this;    }

和StringBuilder的append方法

`
public StringBuilder append(char c) {
append0(c);
return this;
}

这就是一个线程安全而另一个线程不安全的原因`