如何在log4j中创建自己的Appender？

Question

我是log4j的新手.任何人都可以解释如何创建自己的Appender？即如何实现类和接口以及如何覆盖它？

Answer 1

您应该扩展AppenderSkeleton类,(引用javadoc)"提供常用功能的代码,例如支持阈值过滤和支持常规过滤器."

如果您阅读了AppenderSkeleton的代码,您会发现它几乎可以处理所有代码,只留下:

1. protected void append(LoggingEvent事件)
2. public void close()
3. public boolean requiresLayout()

核心方法是追加.请记住,您不需要在其中实现过滤逻辑,因为它已经在doAppend中实现,而doAppend又调用append.在这里,我创建了一个(相当无用的)类,它将日志条目存储在ArrayList中,就像演示一样.

public /*static*/ class MyAppender extends AppenderSkeleton {
    ArrayList<LoggingEvent> eventsList = new ArrayList();

    @Override
    protected void append(LoggingEvent event) {
        eventsList.add(event);
    }

    public void close() {
    }

    public boolean requiresLayout() {
        return false;
    }

}

好的,我们来测试一下:

public static void main (String [] args) {

    Logger l = Logger.getLogger("test");

    MyAppender app = new MyAppender();

    l.addAppender(app);

    l.warn("first");
    l.warn("second");
    l.warn("third");

    l.trace("fourth shouldn't be printed");

    for (LoggingEvent le: app.eventsList) {
        System.out.println("***" + le.getMessage());
    }
} 

你应该打印"第一","第二","第三"; 不应打印第四条消息,因为根记录器的日志级别是调试,而事件级别是跟踪.这证明AbstractSkeleton正确地为我们实现了"级别管理".所以这肯定是要走的路......现在的问题是:为什么你需要一个自定义的appender,而有许多内置的日志到几乎任何目的地？(顺便说一下从log4j开始的好地方:http://logging.apache.org/log4j/1.2/manual.html)

Answer 2

如果你想做一些操纵或决定,你可以这样做:

@Override
protected void append(LoggingEvent event) {
        String message = null;
        if(event.locationInformationExists()){
            StringBuilder formatedMessage = new StringBuilder();
            formatedMessage.append(event.getLocationInformation().getClassName());
            formatedMessage.append(".");
            formatedMessage.append(event.getLocationInformation().getMethodName());
            formatedMessage.append(":");
            formatedMessage.append(event.getLocationInformation().getLineNumber());
            formatedMessage.append(" - ");
            formatedMessage.append(event.getMessage().toString());
            message = formatedMessage.toString();
        }else{
            message = event.getMessage().toString();
        }

        switch(event.getLevel().toInt()){
        case Level.INFO_INT:
            //your decision
            break;
        case Level.DEBUG_INT: 
            //your decision
            break;
        case Level.ERROR_INT:
            //your decision
            break;
        case Level.WARN_INT:
            //your decision
            break;
        case Level.TRACE_INT:
            //your decision
            break;
        default:
            //your decision
            break;
        }
}

Answer 3

我想花费 @AgostinoX 答案来支持 pro 文件配置以及启动和停止日志记录捕获的能力：

public class StringBufferAppender extends org.apache.log4j.AppenderSkeleton {

    StringBuffer logs = new StringBuffer();
    AtomicBoolean captureMode = new AtomicBoolean(false);

    public void close() {
        // TODO Auto-generated method stub

    }

    public boolean requiresLayout() {
        // TODO Auto-generated method stub
        return false;
    }


    @Override
    protected void append(LoggingEvent event) {
        if(captureMode.get())
            logs.append(event.getMessage());
    }

    public void start()
    {
        //System.out.println("[StringBufferAppender|start] - Start capturing logs");
        StringBuffer logs = new StringBuffer();
        captureMode.set(true);
    }

    public StringBuffer stop()
    {
        //System.out.println("[StringBufferAppender|start] - Stop capturing logs");
        captureMode.set(false);
        StringBuffer data = new StringBuffer(logs);
        logs = null;
        return data;
    }


}

现在您所要做的就是在 log4j.property 文件中定义

log4j.rootLogger=...., myAppender  # here you adding your appendr name
log4j.appender.myAppender=com.roi.log.StringBufferAppender # pointing it to the implementation

比您想要在运行期间启用它时更重要的是：

Logger logger = Logger.getRootLogger();
        StringBufferAppender appender = (StringBufferAppender)logger.getAppender("myAppender");
        appender.start();

当想要阻止它时：

StringBuffer sb = appender.stop();