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slf4j 是一个日志规范框架；基本上所有的 JAVA 日志都要实现这个规范；比如：Logback、log4j、log4j2；本文档记载如何实现 slf4j 规范；实现自己的日志框架；

slf4j 分为两个部分，其中包含 ：1.7.x 及其以前的版本、1.7.x 以后的版本；之所以这么说；是因为他们区分注册方式，前面的版本使用固定的类路径进行注册，后面版本使用 JAVA SPI 进行注册

1. 重写日志实现

实现日志打印类，包含了日志输出的逻辑方法：

package com.maple.logger;import org.slf4j.Logger;import java.io.Serializable;public final class MapleLogger implements Logger, Serializable {private final String loggerName;public MapleLogger(String loggerName) {this.loggerName = loggerName;}@Overridepublic String getName() {return this.loggerName;}
}

实现日志打印类获取工厂：

package com.maple.logger;import org.slf4j.ILoggerFactory;
import java.util.concurrent.ConcurrentHashMap;
import java.util.concurrent.ConcurrentMap;public class MapleLoggerFactory implements ILoggerFactory {private final ConcurrentMap<String, MapleLogger> loggerMap = new ConcurrentHashMap<>();@Overridepublic MapleLogger getLogger(String name) {MapleLogger logger = loggerMap.get(name);if (logger != null) {return logger;} else {MapleLogger newInstance = new MapleLogger(name);MapleLogger oldInstance = loggerMap.putIfAbsent(name, newInstance);return oldInstance == null ? newInstance : oldInstance;}}
}

2. 日志绑定

这里才是不同版本的区别，负责把日志框架绑定到运行的程序上下文中：

首先说 1.7.x 之前的版本绑定方法：

创建一个绑定类，用于返回日志工厂实力，但是此类路径必须固定死为 org.slf4j.impl.StaticLoggerBinder 如果不是这个全限定名，则无法进行绑定

package org.slf4j.impl;import com.maple.logger.MapleLoggerFactory;
import org.slf4j.ILoggerFactory;public class StaticLoggerBinder implements org.slf4j.spi.LoggerFactoryBinder {private static StaticLoggerBinder SINGLETON = new StaticLoggerBinder();public static String REQUESTED_API_VERSION = "1.0";private static final String loggerFactoryClassStr = MapleLoggerFactory.class.getName();private final ILoggerFactory loggerFactory;private StaticLoggerBinder() {loggerFactory = new MapleLoggerFactory();}@Overridepublic ILoggerFactory getLoggerFactory() {return loggerFactory;}@Overridepublic String getLoggerFactoryClassStr() {return loggerFactoryClassStr;}public static StaticLoggerBinder getSingleton() {return SINGLETON;}
}

再说 1.7.x 之后的版本绑定方法：

这里采用 SPI 方式进行加载日志绑定类，首先创建日志绑定类：

package com.maple.logger;import org.slf4j.ILoggerFactory;
import org.slf4j.IMarkerFactory;
import org.slf4j.helpers.BasicMDCAdapter;
import org.slf4j.helpers.BasicMarkerFactory;
import org.slf4j.spi.MDCAdapter;
import org.slf4j.spi.SLF4JServiceProvider;public class MapleSLF4JServiceProvider implements SLF4JServiceProvider {public static String REQUESTED_API_VERSION = "2.0.16";private ILoggerFactory loggerFactory;private IMarkerFactory markerFactory;           // 可以根据自己需求进行重写private MDCAdapter mdcAdapter;                  // 可以根据自己需求进行重写@Overridepublic void initialize() {loggerFactory = new MapleLoggerFactory();markerFactory = new BasicMarkerFactory();mdcAdapter = new BasicMDCAdapter();}@Overridepublic ILoggerFactory getLoggerFactory() {return loggerFactory;}@Overridepublic IMarkerFactory getMarkerFactory() {return markerFactory;}@Overridepublic MDCAdapter getMDCAdapter() {return mdcAdapter;}@Overridepublic String getRequestedApiVersion() {return REQUESTED_API_VERSION;}
}

然后添加 SPI 文件，在 resources/META-INF/services 下创建 org.slf4j.spi.SLF4JServiceProvider 文件，文件内容即上面方法的全限定名