标签：tee static gsl 门面模式 new lang contains opera mes
所谓门面模式：提供一个统一接口给用户去访问多个子系统的多个不同接口；
优点：
1.子系统之间解耦
2.简化用户对子系统的使用
3.子系统易扩展增加
缺点：
子系统实现与客户程序存在很大依赖性，需预制客户程序的功能
下以slf4j 为例分析其门面模式实现原理
1. 门面架构分析
2. 源码实现分析
1> 用户通过 Logger logger = LoggerFactory.getLogger(TestMain.class) 获取Logger实例
2> slf4j 通过门面类LoggerFactory查找子系统的实现，可以debug 跟踪 slf4j 源码
public static Logger getLogger(String name) {
ILoggerFactory iLoggerFactory = getILoggerFactory();
return iLoggerFactory.getLogger(name);
}
3> 通过实例类路径"org/slf4j/impl/StaticLoggerBinder.class" 查找加载实现类StaticLoggerBinder(发现多个子系统实现则由JVM选择加载实现类)， 通过单例获取子系统实现类LoggerFactory ，未找到实现类则加载NOP类，修改初始化状态。
private final static void bind() {
try {
Set<URL> staticLoggerBinderPathSet = null;
// skip check under android, see also
// http://jira.qos.ch/browse/SLF4J-328
if (!isAndroid()) {
staticLoggerBinderPathSet = findPossibleStaticLoggerBinderPathSet();
reportMultipleBindingAmbiguity(staticLoggerBinderPathSet);
}
// the next line does the binding
StaticLoggerBinder.getSingleton();
INITIALIZATION_STATE = SUCCESSFUL_INITIALIZATION;
reportActualBinding(staticLoggerBinderPathSet);
fixSubstituteLoggers();
replayEvents();
// release all resources in SUBST_FACTORY
SUBST_FACTORY.clear();
} catch (NoClassDefFoundError ncde) {
String msg = ncde.getMessage();
if (messageContainsOrgSlf4jImplStaticLoggerBinder(msg)) {
INITIALIZATION_STATE = NOP_FALLBACK_INITIALIZATION;
Util.report("Failed to load class \"org.slf4j.impl.StaticLoggerBinder\".");
Util.report("Defaulting to no-operation (NOP) logger implementation");
Util.report("See " + NO_STATICLOGGERBINDER_URL + " for further details.");
} else {
failedBinding(ncde);
throw ncde;
}
} catch (java.lang.NoSuchMethodError nsme) {
String msg = nsme.getMessage();
if (msg != null && msg.contains("org.slf4j.impl.StaticLoggerBinder.getSingleton()")) {
INITIALIZATION_STATE = FAILED_INITIALIZATION;
Util.report("slf4j-api 1.6.x (or later) is incompatible with this binding.");
Util.report("Your binding is version 1.5.5 or earlier.");
Util.report("Upgrade your binding to version 1.6.x.");
}
throw nsme;
} catch (Exception e) {
failedBinding(e);
throw new IllegalStateException("Unexpected initialization failure", e);
}
}
4> 至此完成绑定子系统日志实现框架
后续会更新 slf4j + logback 实现方式及部分源码分析
标签：tee static gsl 门面模式 new lang contains opera mes
原文地址：https://www.cnblogs.com/neverm-one/p/9728192.html
