Java基础(四) StringBuffer、StringBuilder原理浅析

StringBuilder与StringBuffer作用就是用来处理字符串,但String类本身也具备很多方法可以用来处理字符串,那么为什么还要引入这两个类呢?

关于String的讲解请看Java基础(三) String深度解析

首先看下面的例子

public static void main(String[] args) {
    String str0 = "hel,lo,wor,l,d";

    long start = System.currentTimeMillis();
    for (int i = 0; i < 100000; i++){
        str0 += i;
    }
    System.out.println(System.currentTimeMillis() - start);

    StringBuilder sb = new StringBuilder("hel,lo,wor,l,d");
    long start1 = System.currentTimeMillis();
    for (int i = 0; i < 100000; i++){
        sb.append(i);
    }
    System.out.println(System.currentTimeMillis() - start1);

    StringBuffer sbf = new StringBuffer("hel,lo,wor,l,d");
    long start2 = System.currentTimeMillis();
    for (int i = 0; i < 100000; i++){
        sbf.append(i);
    }
    System.out.println(System.currentTimeMillis() - start2);
}

上述代码中3处循环完成了同样的功能,字符串拼接,执行的结果如下:

36823
3
4

可以看出执行时间差别太大,为了解决String不擅长的大量字符串拼接这种业务场景,引入了StringBuffer和StringBuilder.

首先我们分析一下为什么String在大量字符串拼接这种场景下这么慢?

因为String本身不可变,我们对String的任何操作都会返回一个新的对象,然后当前String变量指向新的对象,而原来的String对象就会被GC回收,那么在循环中就会大量快速的创建新的对象,大量原来的对象会不断的被GC回收,消耗的时间是非常恐怖的,而且内存占用非常大。

下面我们对比了String、StringBuffer与StringBuilder的区别

StringStringBufferStringBuilder
final修饰,不可继承final修饰,不可继承final修饰,不可继承
字符串常量,创建后不可变字符串变量,可动态修改字符串变量,可动态修改
不存在线程安全问题线程安全,所有public方法由synchronized修改线程不安全
大量字符串拼接效率最低大量字符串拼接效率非常高大量字符串拼接效率最高

StringBuffer与StringBuilder实现非常类似,下面以StringBuilder简单说明一下append()方法基本原理

1. 首先创建一个StringBuilder

StringBuilder sb1 = new StringBuilder();
StringBuilder sb2 = new StringBuilder(100);

StringBuilder对字符串的操作是通过char[]来实现的,通过默认构造器创建的StringBuilder,其内部创建的char[]的默认长度为16,当然可以调用重载的构造器传递初始长度(推荐这样,因为这样可以减少数组扩容次数,提高效率)。

/**
 * Constructs a string builder with no characters in it and an
 * initial capacity of 16 characters.
 */
public StringBuilder() {
    super(16);
}

2. StringBuilder的append()方法

每次调用append(str)方法时,会首先判断数组长度是否足以添加传递来的字符串

/**
 * Appends the specified string to this character sequence.
 * <p>
 * The characters of the {@code String} argument are appended, in
 * order, increasing the length of this sequence by the length of the
 * argument. If {@code str} is {@code null}, then the four
 * characters {@code "null"} are appended.
 *
 * @param   str   a string.
 * @return  a reference to this object.
 */
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;
}
/**
 * For positive values of {@code minimumCapacity}, this method
 * behaves like {@code ensureCapacity}, however it is never
 * synchronized.
 * If {@code minimumCapacity} is non positive due to numeric
 * overflow, this method throws {@code OutOfMemoryError}.
 */
private void ensureCapacityInternal(int minimumCapacity) {
    // overflow-conscious code
    if (minimumCapacity - value.length > 0) {
        value = Arrays.copyOf(value,
                newCapacity(minimumCapacity));
    }
}

如果传递的字符串长度 + 数组已存放的字符的长度 > 数组的长度,这时就需要进行数据扩容了

/**
 * Returns a capacity at least as large as the given minimum capacity.
 * Returns the current capacity increased by the same amount + 2 if
 * that suffices.
 * Will not return a capacity greater than {@code MAX_ARRAY_SIZE}
 * unless the given minimum capacity is greater than that.
 *
 * @param  minCapacity the desired minimum capacity
 * @throws OutOfMemoryError if minCapacity is less than zero or
 *         greater than Integer.MAX_VALUE
 */
private int newCapacity(int minCapacity) {
    // overflow-conscious code
    int newCapacity = (value.length << 1) + 2;
    if (newCapacity - minCapacity < 0) {
        newCapacity = minCapacity;
    }
    return (newCapacity <= 0 || MAX_ARRAY_SIZE - newCapacity < 0)
        ? hugeCapacity(minCapacity)
        : newCapacity;
}

扩容规则如下:默认将数组长度设置为“ (当前数组长度 * 2) + 2”,但如果按此规则扩容后的数组也不足以添加新的字符串,就需要将数组长度设置为“数组内字符长度 + 传递的字符串长度”。

因此假如我们知道拼接的字符串大概长度有100多字符,我们就可以设置初始长度150或200,这样就可以避免或减少数组扩容的次数,从而提高效率。

总结:

本文StringBuffer与StringBuilder的创建,append方法的原理讲解,对比了String、StringBuffer与StringBuilder异同,若有不对之处,请批评指正,谢谢!

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