slf4j有一个common logger没有的功能，字符串中的{}会被替换，如下：
logger.info("Hello {}","world");
log.debug("debug......");
log.info("info......");
log.error("error......");
这个功能看起来好像很厉害。那实质上slf4j的工程师到底做了什么？会比我们单纯的字符串拼接更快吗？
在slf4j-api:1.7.21这个版本的slf4j的jar中，找到MessageFormatter类，里面有一段代码。
final public static FormattingTuple arrayFormat(final String messagePattern, final Object[] argArray, Throwable throwable) {
if (messagePattern == null) {
return new FormattingTuple(null, argArray, throwable);
}
if (argArray == null) {
return new FormattingTuple(messagePattern);
}
int i = 0;
int j;
// use string builder for better multicore performance
StringBuilder sbuf = new StringBuilder(messagePattern.length() + 50);
int L;
for (L = 0; L < argArray.length; L++) {
j = messagePattern.indexOf(DELIM_STR, i);
if (j == -1) {
// no more variables
if (i == 0) { // this is a simple string
return new FormattingTuple(messagePattern, argArray, throwable);
} else { // add the tail string which contains no variables and return
// the result.
sbuf.append(messagePattern, i, messagePattern.length());
return new FormattingTuple(sbuf.toString(), argArray, throwable);
}
} else {
if (isEscapedDelimeter(messagePattern, j)) {
if (!isDoubleEscaped(messagePattern, j)) {
L--; // DELIM_START was escaped, thus should not be incremented
sbuf.append(messagePattern, i, j - 1);
sbuf.append(DELIM_START);
i = j + 1;
} else {
// The escape character preceding the delimiter start is
// itself escaped: "abc x:\\{}"
// we have to consume one backward slash
sbuf.append(messagePattern, i, j - 1);
deeplyAppendParameter(sbuf, argArray[L], new HashMap<Object[], Object>());
i = j + 2;
}
} else {
// normal case
sbuf.append(messagePattern, i, j);
deeplyAppendParameter(sbuf, argArray[L], new HashMap<Object[], Object>());
i = j + 2;
}
}
}
// append the characters following the last {} pair.
sbuf.append(messagePattern, i, messagePattern.length());
return new FormattingTuple(sbuf.toString(), argArray, throwable);
}
在这个方法中slf4j对{}进行了解析，实质也不是很高效的方法，使用indexOf找到”{}”，再对这个位置处理。简化了以后就是如下这段代码：
final public static FormattingTuple arrayFormat(final String messagePattern, final Object[] argArray, Throwable throwable) {
int i = 0;
int j;
StringBuilder sbuf = new StringBuilder(messagePattern.length() + 50);
int L;
for (L = 0; L < argArray.length; L++) {
j = messagePattern.indexOf("{}", i);
if (j != -1){
sbuf.append(messagePattern,i,j);
sbuf.append(argArray[L]);
i = j+2;
}else{
break;
}
}
sbuf.append(messagePattern, i, messagePattern.length());
return new FormattingTuple(sbuf.toString(), argArray, throwable);
}
而String的indexOf这个方法还是比较消耗性能的。用slf4j这个特性实质上是比我们单纯用logger.info(“Hello “+name);这个方式慢的。于是我做了下实验。用字符串拼接的方式以及StringBuilder的方式去和slf4j的{}进行比较
public String loggerFormat(){
long beginTs = System.currentTimeMillis();
logger.info("loggerFormat begin:"+String.valueOf(beginTs));
for (int i = 0; i < 1000; i++) {
String[] args = new String[]{
getString(),
getString(),
getString(),
getString(),
getString()
};
logger.info("测试{},{},{},{},{}",args);
}
String elapsedTime = "loggerFormat finish:takes "+String.valueOf(System.currentTimeMillis()-beginTs);
logger.info(elapsedTime);
return elapsedTime;
}
public String loggerString(){
long beginTs = System.currentTimeMillis();
logger.info("loggerString begin:"+String.valueOf(beginTs));
for (int i = 0; i < 1000; i++) {
String info = "测试"+getString()+","+getString()+","+getString()+","+getString()+","+getString();
logger.info(info);
}
String elapsedTime = "loggerString finish:takes "+String.valueOf(System.currentTimeMillis()-beginTs);
logger.info(elapsedTime);
return elapsedTime;
}
public String loggerStringBulider(){
long beginTs = System.currentTimeMillis();
logger.info("loggerStringBulider begin:"+String.valueOf(beginTs));
for (int i = 0; i < 1000; i++) {
StringBuilder info = new StringBuilder();
info.append("测试")
.append(getString()).append(",")
.append(getString()).append(",")
.append(getString()).append(",")
.append(getString()).append(",")
.append(getString());
logger.info(info.toString());
}
String elapsedTime = "loggerStringBulider finish:takes "+String.valueOf(System.currentTimeMillis()-beginTs);
logger.info(elapsedTime);
return elapsedTime;
}
public String getString(){
return UUID.randomUUID().toString();
}
得到的结果
2021-01-04 11:42:49.560 [172.27.35.1] [main] INFO c.banger.ubip.authdata.common.LoggerTest - loggerFormat finish:takes 97
2021-01-04 11:42:49.560 [172.27.35.1] [main] INFO c.banger.ubip.authdata.common.LoggerTest - loggerStringBulider finish:takes 66
2021-01-04 11:42:49.560 [172.27.35.1] [main] INFO c.banger.ubip.authdata.common.LoggerTest - loggerString finish:takes 74
最快的是StringBuilder的方式，字符串拼接的方式也比slf4j快。但是，如果是非常多变量的字符串拼接是不可取的，会产生非常多的对象，这时候可以用StringBuilder或者说用slf4j的{}的方式，注意StringBuilder的初始化长度。
