今天在项目里，用了下String.format来格式化字符串，这样写好处时代码量少了那么一点且比较直观，但想想，为啥很少地方有用，是不是性能不好？然后写了以下的代码，比较下常用拼接方法的性能比较。

public class Test{	public static void main(String[] args){		long st = 0l,et=0l;		st= System.nanoTime();		for(int i=0;i<100000;i++){			Test.format("Tim",12);		}		et= System.nanoTime();		System.out.println("format "+(et-st)/1000000+"ms");						st= System.nanoTime();		for(int i=0;i<100000;i++){			Test.plus("Tim",12);		}		et= System.nanoTime();		System.out.println("plus "+(et-st)/1000000+"ms");					st= System.nanoTime();		for(int i=0;i<100000;i++){			Test.concat("Tim",12);		}		et= System.nanoTime();		System.out.println("concat "+(et-st)/1000000+"ms");					st= System.nanoTime();		for(int i=0;i<100000;i++){			Test.builder("Tim",12);		}		et= System.nanoTime();		System.out.println("builder "+(et-st)/1000000+"ms");					st= System.nanoTime();		for(int i=0;i<100000;i++){			Test.buffer("Tim",12);		}		et= System.nanoTime();		System.out.println("buffer "+(et-st)/1000000+"ms");		}		static String format(String name,int age){		return String.format("我叫%s，今年%d岁",name,age);	}		static String plus(String name,int age){		return "我叫"+name+"，今年"+age+"岁";	}		static String concat(String name,int age){		return "我叫".concat(name).concat("，今年").concat(String.valueOf(age)).concat("岁");	}		static String builder(String name,int age){		StringBuilder sb=new StringBuilder();		sb.append("我叫").append(name).append("，今年").append(age).append("岁");		return sb.toString();	}		static String buffer(String name,int age){		StringBuffer sb=new StringBuffer();		sb.append("我叫").append(name).append("，今年").append(age).append("岁");		return sb.toString();	}}

测试结果是：

format 331msplus 21msconcat 18msbuilder 12msbuffer 22ms

除了String.format外，其他都在意料之内。String.format的耗时也太超出想像了吧。

然后看了下实现的代码。

public Formatter format(Locale l, String format, Object ... args) {	ensureOpen();	// index of last argument referenced	int last = -1;	// last ordinary index	int lasto = -1;	FormatString[] fsa = parse(format);	for (int i = 0; i < fsa.length; i++) {	    FormatString fs = fsa[i];	    int index = fs.index();	    try {		switch (index) {		case -2:  // fixed string, "%n", or "%%"		    fs.print(null, l);		    break;		case -1:  // relative index		    if (last < 0 || (args != null && last > args.length - 1))			throw new MissingFormatArgumentException(fs.toString());		    fs.print((args == null ? null : args[last]), l);		    break;		case 0:  // ordinary index		    lasto++; 		    last = lasto;		    if (args != null && lasto > args.length - 1)			throw new MissingFormatArgumentException(fs.toString()); 		    fs.print((args == null ? null : args[lasto]), l);		    break;		default:  // explicit index		    last = index - 1;		    if (args != null && last > args.length - 1)			throw new MissingFormatArgumentException(fs.toString()); 		    fs.print((args == null ? null : args[last]), l);		    break;		}	    } catch (IOException x) {		lastException = x;	    }	}	return this;    }