SLF4J簡介

The Simple Logging Facade for Java (SLF4J) serves as a simple facade or abstraction for various logging frameworks, such as java.util.logging, logback and log4j. SLF4J allows the end-user to plug in the desired logging framework at deployment time.

簡單來說，SLF4J提供了一套日志接口（還是面向接口編程的思想），具體的接口實現是用logback還是log4j或者其他日志框架，要看你部署時使用的jar包。
SLF4J使用

??? import org.slf4j.Logger;
??? import org.slf4j.LoggerFactory;
??? ?
??? public class HelloWorld {
????? public static void main(String[] args) {
??????? Logger logger = LoggerFactory.getLogger(HelloWorld.class);
??????? logger.info("Hello World");
????? }
??? }

要把這個程序跑起來需要到SLF4J官網下載jar包，當前最新的是slf4j-api-1.7.7.jar，然后需要再選擇以下jar包來綁定具體的日志框架：

??? slf4j-log4j12-1.7.7.jar
??? slf4j-jdk14-1.7.7.jar
??? slf4j-nop-1.7.7.jar
??? slf4j-simple-1.7.7.jar
??? slf4j-jcl-1.7.7.jar
??? logback-classic-1.0.13.jar （非SLF4J項目提供，不推薦）

需要切換到不同日志框架時，只需替換上述jar包即可。

貼上一張SLF4J框架圖幫助理解，

實現原理

這么碉堡的功能，是怎么做到的呢？說來簡單，但又很奇葩，先看一下LoggerFactory.getLogger，

????? /**
?????? * Return a logger named according to the name parameter using the statically
?????? * bound {@link ILoggerFactory} instance.
?????? *
?????? * @param name The name of the logger.
?????? * @return logger
?????? */
????? public static Logger getLogger(String name) {
??????? ILoggerFactory iLoggerFactory = getILoggerFactory();
??????? return iLoggerFactory.getLogger(name);
????? }

調用了getILoggerFactory，

????? /**
?????? * Return the {@link ILoggerFactory} instance in use.
?????? * <p/>
?????? * <p/>
?????? * ILoggerFactory instance is bound with this class at compile time.
?????? *
?????? * @return the ILoggerFactory instance in use
?????? */
????? public static ILoggerFactory getILoggerFactory() {
??????? if (INITIALIZATION_STATE == UNINITIALIZED) {
????????? INITIALIZATION_STATE = ONGOING_INITIALIZATION;
????????? performInitialization();
??????? }
??????? switch (INITIALIZATION_STATE) {
????????? case SUCCESSFUL_INITIALIZATION:
??????????? return StaticLoggerBinder.getSingleton().getLoggerFactory();
????????? case NOP_FALLBACK_INITIALIZATION:
??????????? return NOP_FALLBACK_FACTORY;
????????? case FAILED_INITIALIZATION:
??????????? throw new IllegalStateException(UNSUCCESSFUL_INIT_MSG);
????????? case ONGOING_INITIALIZATION:
??????????? // support re-entrant behavior.
??????????? // See also http://bugzilla.slf4j.org/show_bug.cgi?id=106
??????????? return TEMP_FACTORY;
??????? }
??????? throw new IllegalStateException("Unreachable code");
????? }

繼續跟下去可以看到最后是通過StaticLoggerBinder綁定到具體的日志框架，但是這個StaticLoggerBinder很神奇，看下圖

通過對比SLF4J的jar包跟源碼包發現，源碼包竟然多出了一個impl目錄，此目錄有以下幾個文件，

??? package.html
??? StaticLoggerBinder.java
??? StaticMarkerBinder.java
??? StaticMDCBinder.java

看一下StaticLoggerBinder.java，

??? public class StaticLoggerBinder {
??? ?
????? /**
?????? * The unique instance of this class.
?????? */
????? private static final StaticLoggerBinder SINGLETON = new StaticLoggerBinder();
???? ?
????? /**
?????? * Return the singleton of this class.
?????? *
?????? * @return the StaticLoggerBinder singleton
?????? */
????? public static final StaticLoggerBinder getSingleton() {
??????? return SINGLETON;
????? }
???? ?
????? /**
?????? * Declare the version of the SLF4J API this implementation is compiled against.
?????? * The value of this field is usually modified with each release.
?????? */
????? // to avoid constant folding by the compiler, this field must *not* be final
????? public static String REQUESTED_API_VERSION = "1.6.99";? // !final
???? ?
????? private StaticLoggerBinder() {
??????? throw new UnsupportedOperationException("This code should have never made it into slf4j-api.jar");
????? }
??? ?
????? public ILoggerFactory getLoggerFactory() {
??????? throw new UnsupportedOperationException("This code should never make it into slf4j-api.jar");
????? }
??? ?
????? public String getLoggerFactoryClassStr() {
??????? throw new UnsupportedOperationException("This code should never make it into slf4j-api.jar");
????? }
??? }

"This code should have never made it into slf4j-api.jar"，想來想去，只有一種可能，就是打完jar包后，手工將impl這個目錄從jar包中刪除了。而在上述slf4j-log4j12-1.7.7.jar等包中，你都可以找到org.slf4j.impl.StaticLoggerBinder這個類。

到這里應該可以看出來，這是一種很奇葩的擴展方式，先使用了一個類，將它打進jar包，然后從jar包中剔除，通過在其他jar包中使用同樣類名新建一個類（至于這個類具體用來做啥并無限制，也沒有接口來約束）來達到擴展的效果。目前看來這樣的方式有兩個小問題：

??? 需要防止一些編譯器優化。例如上面看到的StaticLoggerBinder.REQUESTED_API_VERSION，為防止編譯器的constant folding，不能聲明為final。
??? 對于被擴展的類約束太少。為了解決這個問題，SLF4J專門定義了一個org.slf4j.spi.LoggerFactoryBinder，具體實現的org.slf4j.impl.StaticLoggerBinde都遵循了這個接口，但這其實只是一種約定，而不是約束。

使用反射？

今天某同學說，為嘛不直接反射出一個接口，例如LoggerFactoryBinder，我想了想，ServiceLoader機制不就是使用反射的嗎，SLF4J真是不走尋常路^_^
參考資料

??? http://www.slf4j.org/manual.html

?

---------------------
作者：kisimple
來源：CSDN
原文：https://blog.csdn.net/kisimple/article/details/38664717
版權聲明：本文為博主原創文章，轉載請附上博文鏈接！

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