Java字符串拼接的五種方法，哪種性能最好？
作者 | 老壇酸菜WH
> 字符串拼接一般使用“+”，但是“+”不能滿足大批量數(shù)據(jù)的處理，Java中有以下五種方法處理字符串拼接，各有優(yōu)缺點(diǎn)，程序開(kāi)發(fā)應(yīng)選擇合適的方法實(shí)現(xiàn)。
加號(hào) “+”
String contact() 方法
StringUtils.join() 方法
StringBuffer append() 方法
StringBuilder append() 方法
> 經(jīng)過(guò)簡(jiǎn)單的程序測(cè)試，從執(zhí)行100次到90萬(wàn)次的時(shí)間開(kāi)銷如下表：
由此可以看出：
方法1 加號(hào) “+” 拼接 和 方法2 String contact() 方法 適用于小數(shù)據(jù)量的操作，代碼簡(jiǎn)潔方便，加號(hào)“+” 更符合我們的編碼和閱讀習(xí)慣；
方法3 StringUtils.join() 方法 適用于將ArrayList轉(zhuǎn)換成字符串，就算90萬(wàn)條數(shù)據(jù)也只需68ms，可以省掉循環(huán)讀取ArrayList的代碼；
方法4 StringBuffer append() 方法 和 方法5 StringBuilder append() 方法 其實(shí)他們的本質(zhì)是一樣的，都是繼承自AbstractStringBuilder，效率最高，大批量的數(shù)據(jù)處理最好選擇這兩種方法。
方法1 加號(hào) “+” 拼接 和 方法2 String contact() 方法 的時(shí)間和空間成本都很高（分析在本文末尾），不能用來(lái)做批量數(shù)據(jù)的處理。
> 源代碼，供參考
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");
}
}
執(zhí)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
執(zhí)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
執(zhí)行1萬(wàn)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
執(zhí)行10萬(wàn)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
執(zhí)行20萬(wàn)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
執(zhí)行50萬(wàn)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
執(zhí)行90萬(wàn)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
> 查看源代碼，以及簡(jiǎn)單分析
String contact 和 StringBuffer，StringBuilder 的源代碼都可以在Java庫(kù)里找到，有空可以研究研究。
其實(shí)每次調(diào)用contact()方法就是一次數(shù)組的拷貝，雖然在內(nèi)存中是處理都是原子性操作，速度非?？?，但是，最后的return語(yǔ)句會(huì)創(chuàng)建一個(gè)新String對(duì)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);
}
StringBuffer 和 StringBuilder 的append方法都繼承自AbstractStringBuilder，整個(gè)邏輯都只做字符數(shù)組的加長(zhǎng)，拷貝，到最后也不會(huì)創(chuàng)建新的String對(duì)象，所以速度很快，完成拼接處理后在程序中用strBuffer.toString()來(lái)得到最終的字符串。
/**
* 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);
}
字符串的加號(hào)“+” 方法， 雖然編譯器對(duì)其做了優(yōu)化，使用StringBuilder的append方法進(jìn)行追加，但是每循環(huán)一次都會(huì)創(chuàng)建一個(gè)StringBuilder對(duì)象，且都會(huì)調(diào)用toString方法轉(zhuǎn)換成字符串，所以開(kāi)銷很大。如果您正在學(xué)習(xí)Spring Cloud，推薦一個(gè)連載多年還在繼續(xù)更新的免費(fèi)教程：https://blog.didispace.com/spring-cloud-learning/
注：執(zhí)行一次字符串“+”,相當(dāng)于 str = new StringBuilder(str).append("a").toString();
本文開(kāi)頭的地方統(tǒng)計(jì)了時(shí)間開(kāi)銷，根據(jù)上述分析再想想空間的開(kāi)銷。常說(shuō)拿空間換時(shí)間，反過(guò)來(lái)是不是拿時(shí)間換到了空間呢，但是在這里，其實(shí)時(shí)間是消耗在了重復(fù)的不必要的工作上（生成新的對(duì)象，toString方法），所以對(duì)大批量數(shù)據(jù)做處理時(shí)，加號(hào)“+” 和 contact 方法絕對(duì)不能用，時(shí)間和空間成本都很高。
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