Java字符串拼接的五种方法及性能比较分析(从执行100次到90万次)
> 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 { 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
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); } /** * 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); }
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。
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