STRUTS2 源码 - Logging System

看了STRUTS2的源码，了解了它的logging系统，觉得还是蛮有意思的，用到了很多设计模式。

先看类结构图：

1. 工厂方法模式

这个就不说了，直接明了。

2. 伪单例模式

LoggerFactory是个抽象方法，同时里面也包含了对于LoggerFactory的伪单例实现。为什么是伪单例，因为看起来像是单例模式，但其实你也可以创建多个实例：

    public static void setLoggerFactory(LoggerFactory factory) {
        lock.writeLock().lock();
        try {
            LoggerFactory.factory = factory;
        } finally {
            lock.writeLock().unlock();
        }
            
    }

    protected static LoggerFactory getLoggerFactory() {
        lock.readLock().lock();
        try {
            if (factory != null) {
                return factory;
            }
        } finally {
            lock.readLock().unlock();
        }
        lock.writeLock().lock();
        try {
            if (factory == null) {
                try {
                    Class.forName("org.apache.commons.logging.LogFactory");
                    factory = new com.opensymphony.xwork2.util.logging.commons.CommonsLoggerFactory();
                } catch (ClassNotFoundException ex) {
                    // commons logging not found, falling back to jdk logging
                    factory = new JdkLoggerFactory();
                }
            }
            return factory;
        }
        finally {
            lock.writeLock().unlock();
        }
    }

这里还用到了concurrent包里面的ReentrantReadWriteLock锁，看着使用方法貌似还蛮简单的，以后可以将这个类放到工具箱里。

3. 模板方法模式

LoggerFactory:

    public static Logger getLogger(Class<?> cls) {
        return getLoggerFactory().getLoggerImpl(cls);
    }
    
    public static Logger getLogger(String name) {
        return getLoggerFactory().getLoggerImpl(name);
    }

    protected abstract Logger getLoggerImpl(Class<?> cls);
    
    protected abstract Logger getLoggerImpl(String name);

JdkLoggerFactory:

public class JdkLoggerFactory extends LoggerFactory {

    @Override
    protected Logger getLoggerImpl(Class<?> cls) {
        return new JdkLogger(java.util.logging.Logger.getLogger(cls.getName()));
    }
    
    @Override
    protected Logger getLoggerImpl(String name) {
        return new JdkLogger(java.util.logging.Logger.getLogger(name));
    }
}

4. 代理模式

JDKLogger并没有做任何事情，而是直接代理给了java.util.logging.Logger类：

public class JdkLogger implements Logger {
    
    private java.util.logging.Logger log;
    
    public JdkLogger(java.util.logging.Logger log) {
        this.log = log;
    }

    public void error(String msg, String... args) {
        log.log(Level.SEVERE, LoggerUtils.format(msg, args));
    }

...
}

STRUTS2 源码 - Logging System

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