Java 字符串拼接 五种方法的性能比较分析 从执行100次到90万次

 【请尊重原创版权，如需引用，请注明来源及地址】

> 字符串拼接一般使用“+”，但是“+”不能满足大批量数据的处理，Java中有以下五种方法处理字符串拼接，各有优缺点，程序开发应选择合适的方法实现。

1. 加号 “+”

2. String contact() 方法

3. StringUtils.join() 方法

4. StringBuffer append() 方法

5. StringBuilder append() 方法

> 经过简单的程序测试，从执行100次到90万次的时间开销如下表：

 

 由此可以看出：

1. 方法1 加号 “+” 拼接 和 方法2 String contact() 方法 适用于小数据量的操作，代码简洁方便，加号“+” 更符合我们的编码和阅读习惯；

2. 方法3 StringUtils.join() 方法 适用于将ArrayList转换成字符串，就算90万条数据也只需68ms，可以省掉循环读取ArrayList的代码；

3. 方法4 StringBuffer append() 方法 和 方法5 StringBuilder append() 方法 其实他们的本质是一样的，都是继承自AbstractStringBuilder，效率最高，大批量的数据处理最好选择这两种方法。

4. 方法1 加号 “+” 拼接 和 方法2 String contact() 方法 的时间和空间成本都很高（分析在本文末尾），不能用来做批量数据的处理。

> 源代码，供参考

package cnblogs.twzheng.lab2;
/**

@author Tan Wenzheng


*/

import java.util.ArrayList;

import java.util.List;
import org.apache.commons.lang3.StringUtils;
public class TestString {
</span><span style="color: #0000ff;">private</span> <span style="color: #0000ff;">static</span> <span style="color: #0000ff;">final</span> <span style="color: #0000ff;">int</span> max = 100<span style="color: #000000;">;

</span><span style="color: #0000ff;">public</span> <span style="color: #0000ff;">void</span><span style="color: #000000;"> testPlus() {
    System.out.println(</span>"&gt;&gt;&gt; testPlus() &lt;&lt;&lt;"<span style="color: #000000;">);

    String str </span>= ""<span style="color: #000000;">;

    </span><span style="color: #0000ff;">long</span> start =<span style="color: #000000;"> System.currentTimeMillis();

    </span><span style="color: #0000ff;">for</span> (<span style="color: #0000ff;">int</span> i = 0; i &lt; max; i++<span style="color: #000000;">) {
        str </span>= str + "a"<span style="color: #000000;">;
    }

    </span><span style="color: #0000ff;">long</span> end =<span style="color: #000000;"> System.currentTimeMillis();

    </span><span style="color: #0000ff;">long</span> cost = end -<span style="color: #000000;"> start;

    System.out.println(</span>"   {str + "a"} cost=" + cost + " ms"<span style="color: #000000;">);
}

</span><span style="color: #0000ff;">public</span> <span style="color: #0000ff;">void</span><span style="color: #000000;"> testConcat() {
    System.out.println(</span>"&gt;&gt;&gt; testConcat() &lt;&lt;&lt;"<span style="color: #000000;">);

    String str </span>= ""<span style="color: #000000;">;

    </span><span style="color: #0000ff;">long</span> start =<span style="color: #000000;"> System.currentTimeMillis();

    </span><span style="color: #0000ff;">for</span> (<span style="color: #0000ff;">int</span> i = 0; i &lt; max; i++<span style="color: #000000;">) {
        str </span>= str.concat("a"<span style="color: #000000;">);
    }

    </span><span style="color: #0000ff;">long</span> end =<span style="color: #000000;"> System.currentTimeMillis();

    </span><span style="color: #0000ff;">long</span> cost = end -<span style="color: #000000;"> start;

    System.out.println(</span>"   {str.concat("a")} cost=" + cost + " ms"<span style="color: #000000;">);
}

</span><span style="color: #0000ff;">public</span> <span style="color: #0000ff;">void</span><span style="color: #000000;"> testJoin() {
    System.out.println(</span>"&gt;&gt;&gt; testJoin() &lt;&lt;&lt;"<span style="color: #000000;">);

    </span><span style="color: #0000ff;">long</span> start =<span style="color: #000000;"> System.currentTimeMillis();

    List</span>&lt;String&gt; list = <span style="color: #0000ff;">new</span> ArrayList&lt;String&gt;<span style="color: #000000;">();

    </span><span style="color: #0000ff;">for</span> (<span style="color: #0000ff;">int</span> i = 0; i &lt; max; i++<span style="color: #000000;">) {
        list.add(</span>"a"<span style="color: #000000;">);
    }

    </span><span style="color: #0000ff;">long</span> end1 =<span style="color: #000000;"> System.currentTimeMillis();
    </span><span style="color: #0000ff;">long</span> cost1 = end1 -<span style="color: #000000;"> start;

    StringUtils.join(list, </span>""<span style="color: #000000;">);

    </span><span style="color: #0000ff;">long</span> end =<span style="color: #000000;"> System.currentTimeMillis();
    </span><span style="color: #0000ff;">long</span> cost = end -<span style="color: #000000;"> end1;

    System.out.println(</span>"   {list.add("a")} cost1=" + cost1 + " ms"<span style="color: #000000;">);
    System.out.println(</span>"   {StringUtils.join(list, "")} cost=" +<span style="color: #000000;"> cost
            </span>+ " ms"<span style="color: #000000;">);
}

</span><span style="color: #0000ff;">public</span> <span style="color: #0000ff;">void</span><span style="color: #000000;"> testStringBuffer() {
    System.out.println(</span>"&gt;&gt;&gt; testStringBuffer() &lt;&lt;&lt;"<span style="color: #000000;">);

    </span><span style="color: #0000ff;">long</span> start =<span style="color: #000000;"> System.currentTimeMillis();

    StringBuffer strBuffer </span>= <span style="color: #0000ff;">new</span><span style="color: #000000;"> StringBuffer();

    </span><span style="color: #0000ff;">for</span> (<span style="color: #0000ff;">int</span> i = 0; i &lt; max; i++<span style="color: #000000;">) {
        strBuffer.append(</span>"a"<span style="color: #000000;">);
    }
    strBuffer.toString();

    </span><span style="color: #0000ff;">long</span> end =<span style="color: #000000;"> System.currentTimeMillis();

    </span><span style="color: #0000ff;">long</span> cost = end -<span style="color: #000000;"> start;

    System.out.println(</span>"   {strBuffer.append("a")} cost=" + cost + " ms"<span style="color: #000000;">);
}

</span><span style="color: #0000ff;">public</span> <span style="color: #0000ff;">void</span><span style="color: #000000;"> testStringBuilder() {
    System.out.println(</span>"&gt;&gt;&gt; testStringBuilder() &lt;&lt;&lt;"<span style="color: #000000;">);

    </span><span style="color: #0000ff;">long</span> start =<span style="color: #000000;"> System.currentTimeMillis();

    StringBuilder strBuilder </span>= <span style="color: #0000ff;">new</span><span style="color: #000000;"> StringBuilder();

    </span><span style="color: #0000ff;">for</span> (<span style="color: #0000ff;">int</span> i = 0; i &lt; max; i++<span style="color: #000000;">) {
        strBuilder.append(</span>"a"<span style="color: #000000;">);
    }
    strBuilder.toString();

    </span><span style="color: #0000ff;">long</span> end =<span style="color: #000000;"> System.currentTimeMillis();

    </span><span style="color: #0000ff;">long</span> cost = end -<span style="color: #000000;"> start;

    System.out
            .println(</span>"   {strBuilder.append("a")} cost=" + cost + " ms"<span style="color: #000000;">);
}

}

> 测试结果：

1. 执行100次， private static final int max = 100;

>>> testPlus() <<<
   {str + "a"} cost=0 ms
>>> testConcat() <<<
   {str.concat("a")} cost=0 ms
>>> testJoin() <<<
   {list.add("a")} cost1=0 ms
   {StringUtils.join(list, "")} cost=20 ms
>>> testStringBuffer() <<<
   {strBuffer.append("a")} cost=0 ms
>>> testStringBuilder() <<<
   {strBuilder.append("a")} cost=0 ms

2. 执行1000次， private static final int max = 1000;

>>> testPlus() <<<
   {str + "a"} cost=10 ms
>>> testConcat() <<<
   {str.concat("a")} cost=0 ms
>>> testJoin() <<<
   {list.add("a")} cost1=0 ms
   {StringUtils.join(list, "")} cost=20 ms
>>> testStringBuffer() <<<
   {strBuffer.append("a")} cost=0 ms
>>> testStringBuilder() <<<
   {strBuilder.append("a")} cost=0 ms

3. 执行1万次， private static final int max = 10000;

>>> testPlus() <<<
   {str + "a"} cost=150 ms
>>> testConcat() <<<
   {str.concat("a")} cost=70 ms
>>> testJoin() <<<
   {list.add("a")} cost1=0 ms
   {StringUtils.join(list, "")} cost=30 ms
>>> testStringBuffer() <<<
   {strBuffer.append("a")} cost=0 ms
>>> testStringBuilder() <<<
   {strBuilder.append("a")} cost=0 ms

4. 执行10万次， private static final int max = 100000;

>>> testPlus() <<<
   {str + "a"} cost=4198 ms
>>> testConcat() <<<
   {str.concat("a")} cost=1862 ms
>>> testJoin() <<<
   {list.add("a")} cost1=21 ms
   {StringUtils.join(list, "")} cost=49 ms
>>> testStringBuffer() <<<
   {strBuffer.append("a")} cost=10 ms
>>> testStringBuilder() <<<
   {strBuilder.append("a")} cost=10 ms

5. 执行20万次， private static final int max = 200000;

>>> testPlus() <<<
   {str + "a"} cost=17196 ms
>>> testConcat() <<<
   {str.concat("a")} cost=7653 ms
>>> testJoin() <<<
   {list.add("a")} cost1=20 ms
   {StringUtils.join(list, "")} cost=51 ms
>>> testStringBuffer() <<<
   {strBuffer.append("a")} cost=20 ms
>>> testStringBuilder() <<<
   {strBuilder.append("a")} cost=16 ms

6. 执行50万次， private static final int max = 500000;

>>> testPlus() <<<
   {str + "a"} cost=124693 ms
>>> testConcat() <<<
   {str.concat("a")} cost=49439 ms
>>> testJoin() <<<
   {list.add("a")} cost1=21 ms
   {StringUtils.join(list, "")} cost=50 ms
>>> testStringBuffer() <<<
   {strBuffer.append("a")} cost=20 ms
>>> testStringBuilder() <<<
   {strBuilder.append("a")} cost=10 ms

7. 执行90万次， private static final int max = 900000;

>>> testPlus() <<<
   {str + "a"} cost=456739 ms
>>> testConcat() <<<
   {str.concat("a")} cost=186252 ms
>>> testJoin() <<<
   {list.add("a")} cost1=20 ms
   {StringUtils.join(list, "")} cost=68 ms
>>> testStringBuffer() <<<
   {strBuffer.append("a")} cost=30 ms
>>> testStringBuilder() <<<
   {strBuilder.append("a")} cost=24 ms

> 查看源代码，以及简单分析

String contact 和 StringBuffer，StringBuilder 的源代码都可以在Java库里找到，有空可以研究研究。

1. 其实每次调用contact()方法就是一次数组的拷贝，虽然在内存中是处理都是原子性操作，速度非常快，但是，最后的return语句会创建一个新String对象，限制了concat方法的速度。

    public String concat(String str) {
        int otherLen = str.length();
        if (otherLen == 0) {
            return this;
        }
        int len = value.length;
        char buf[] = Arrays.copyOf(value, len + otherLen);
        str.getChars(buf, len);
        return new String(buf, true);
    }

2. StringBuffer 和 StringBuilder 的append方法都继承自AbstractStringBuilder，整个逻辑都只做字符数组的加长，拷贝，到最后也不会创建新的String对象，所以速度很快，完成拼接处理后在程序中用strBuffer.toString()来得到最终的字符串。

    /**
     * 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.
     * <p>
     * Let <i>n</i> be the length of this character sequence just prior to
     * execution of the {@code append} method. Then the character at
     * index <i>k</i> in the new character sequence is equal to the character
     * at index <i>k</i> in the old character sequence, if <i>k</i> is less
     * than <i>n</i>; otherwise, it is equal to the character at index
     * <i>k-n</i> in the argument {@code str}.
     *
     * @param   str   a string.
     * @return  a reference to this object.
     */
    public AbstractStringBuilder append(String str) {
        if (str == null) str = "null";
        int len = str.length();
        ensureCapacityInternal(count + len);
        str.getChars(0, len, value, count);
        count += len;
        return this;
    }

    /**
     * This method has the same contract as ensureCapacity, but is
     * never synchronized.
     */
    private void ensureCapacityInternal(int minimumCapacity) {
        // overflow-conscious code
        if (minimumCapacity - value.length > 0)
            expandCapacity(minimumCapacity);
    }
</span><span style="color: #008000;">/**</span><span style="color: #008000;">
 * This implements the expansion semantics of ensureCapacity with no
 * size check or synchronization.
 </span><span style="color: #008000;">*/</span>
<span style="color: #0000ff;">void</span> expandCapacity(<span style="color: #0000ff;">int</span><span style="color: #000000;"> minimumCapacity) {
    </span><span style="color: #0000ff;">int</span> newCapacity = value.length * 2 + 2<span style="color: #000000;">;
    </span><span style="color: #0000ff;">if</span> (newCapacity - minimumCapacity &lt; 0<span style="color: #000000;">)
        newCapacity </span>=<span style="color: #000000;"> minimumCapacity;
    </span><span style="color: #0000ff;">if</span> (newCapacity &lt; 0<span style="color: #000000;">) {
        </span><span style="color: #0000ff;">if</span> (minimumCapacity &lt; 0) <span style="color: #008000;">//</span><span style="color: #008000;"> overflow</span>
            <span style="color: #0000ff;">throw</span> <span style="color: #0000ff;">new</span><span style="color: #000000;"> OutOfMemoryError();
        newCapacity </span>=<span style="color: #000000;"> Integer.MAX_VALUE;
    }
    value </span>=<span style="color: #000000;"> Arrays.copyOf(value, newCapacity);
}</span></pre>

3. 字符串的加号“+” 方法， 虽然编译器对其做了优化，使用StringBuilder的append方法进行追加，但是每循环一次都会创建一个StringBuilder对象，且都会调用toString方法转换成字符串，所以开销很大。

　　注：执行一次字符串“+”,相当于 str = new StringBuilder(str).append("a").toString();

4. 本文开头的地方统计了时间开销，根据上述分析再想想空间的开销。常说拿空间换时间，反过来是不是拿时间换到了空间呢，但是在这里，其实时间是消耗在了重复的不必要的工作上（生成新的对象，toString方法），所以对大批量数据做处理时，加号“+” 和 contact 方法绝对不能用，时间和空间成本都很高。

 【请尊重原创版权，如需引用，请注明来源及地址】