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。

Java字符串拼接的五种方法，哪种性能最好？_字符串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：

Java字符串拼接的五种方法，哪种性能最好？_字符串_02

​​由此可以看出：​​​​
​​Java字符串拼接的五种方法，哪种性能最好？_i++_03

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 {

    private static final int max = 100;

    public void testPlus() {
        System.out.println(">>> testPlus() <<<");

String str = "";

long start = System.currentTimeMillis();

for (int i = 0; i < max; i++) {
str = str + "a";
}

long end = System.currentTimeMillis();

long cost = end - start;

System.out.println("   {str + \"a\"} cost=" + cost + " ms");
}

public void testConcat() {
System.out.println(">>> testConcat() <<<");

String str = "";

long start = System.currentTimeMillis();

for (int i = 0; i < max; i++) {
str = str.concat("a");
}

long end = System.currentTimeMillis();

long cost = end - start;

System.out.println("   {str.concat(\"a\")} cost=" + cost + " ms");
}

public void testJoin() {
System.out.println(">>> testJoin() <<<");

long start = System.currentTimeMillis();

List<String> list = new ArrayList<String>();

for (int i = 0; i < max; i++) {
list.add("a");
}

long end1 = System.currentTimeMillis();
long cost1 = end1 - start;

StringUtils.join(list, "");

long end = System.currentTimeMillis();
long cost = end - end1;

System.out.println("   {list.add(\"a\")} cost1=" + cost1 + " ms");
System.out.println("   {StringUtils.join(list, \"\")} cost=" + cost
+ " ms");
}

public void testStringBuffer() {
System.out.println(">>> testStringBuffer() <<<");

long start = System.currentTimeMillis();

StringBuffer strBuffer = new StringBuffer();

for (int i = 0; i < max; i++) {
strBuffer.append("a");
}
strBuffer.toString();

long end = System.currentTimeMillis();

long cost = end - start;

System.out.println("   {strBuffer.append(\"a\")} cost=" + cost + " ms");
}

public void testStringBuilder() {
System.out.println(">>> testStringBuilder() <<<");

long start = System.currentTimeMillis();

StringBuilder strBuilder = new StringBuilder();

for (int i = 0; i < max; i++) {
strBuilder.append("a");
}
strBuilder.toString();

long end = System.currentTimeMillis();

long cost = end - start;

System.out
.println("   {strBuilder.append(\"a\")} cost=" + cost + " ms");
}
}

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
  1. 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
  1. 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
  1. 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
  1. 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
  1. 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
  1. 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);
    }
  1. 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);
    }

    /**
     * This implements the expansion semantics of ensureCapacity with no
     * size check or synchronization.
     */
    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);
}
  1. 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();

  1. 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。


Java字符串拼接的五种方法，哪种性能最好？_i++_04