Appender.appender是空的，基于MDCSiftingAppender in log4j.properties (log4j 1)

Question

我得到了一个正在工作的java项目。它使用log4j (而不是log4j 2)登录。

我想在不添加log4j 2或任何其他将破坏现有log4依赖项的依赖项的情况下添加一个sift附录程序。

我有这个log4j.properties文件

# Root logger
log4j.rootLogger=INFO, sift
log4j.throwableRenderer=org.apache.log4j.OsgiThrowableRenderer
log4j.configDebug = true

# Sift appender
log4j.appender.sift=org.apache.log4j.sift.MDCSiftingAppender
log4j.appender.sift.key=session_id
log4j.appender.sift.default=no_session_id
log4j.appender.sift.appender=org.apache.log4j.FileAppender
log4j.appender.sift.appender.layout=org.apache.log4j.PatternLayout
log4j.appender.sift.appender.layout.ConversionPattern=%d{ISO8601} | %-5.5p | %-16.16t | %-32.32c{1} | %m%n
log4j.appender.sift.appender.file=/var/log/rr/$\\{session_id\\}.log
log4j.appender.sift.appender.append=true

这是我的主要：

public class SiftExampleLog4j {

    static org.apache.log4j.Logger logger = Logger.getLogger(SiftExampleLog4j.class);

    public void log() {
        BasicConfigurator.configure();
        org.apache.log4j.MDC.put("session_id","MyGooApp");

        logger.error("example1");

        org.apache.log4j.MDC.put("session_id","MyFooApp");

        logger.error("example2");
    }

    public static void main (String[] args){
    new SiftExampleLog4j().log();
    }
}

但是当我尝试登录时，我得到了NPE

logger.error("example1");

我错过了什么，如何使“围裙”成员不为零？

下面是缩写main的堆栈跟踪

Exception in thread "main" java.lang.NullPointerException
    at org.apache.log4j.sift.MDCSiftingAppender.getAppender(MDCSiftingAppender.java:109)
    at org.apache.log4j.sift.MDCSiftingAppender.append(MDCSiftingAppender.java:79)
    at org.apache.log4j.AppenderSkeleton.doAppend(AppenderSkeleton.java:251)
    at org.apache.log4j.helpers.AppenderAttachableImpl.appendLoopOnAppenders(AppenderAttachableImpl.java:66)
    at org.apache.log4j.Category.callAppenders(Category.java:206)
    at org.apache.log4j.Category.forcedLog(Category.java:391)
    at org.apache.log4j.Category.error(Category.java:305)
    at com.waze.rr_logger.SiftExampleLog4j.log(SiftExampleLog4j.java:14)
    at com.waze.rr_logger.SiftExampleLog4j.main(SiftExampleLog4j.java:22)
    at sun.reflect.NativeMethodAccessorImpl.invoke0(Native Method)
    at sun.reflect.NativeMethodAccessorImpl.invoke(NativeMethodAccessorImpl.java:62)
    at sun.reflect.DelegatingMethodAccessorImpl.invoke(DelegatingMethodAccessorImpl.java:43)
    at java.lang.reflect.Method.invoke(Method.java:498)
    at com.intellij.rt.execution.application.AppMain.main(AppMain.java:144)

但是当我尝试登录时，我得到了NPE

我遗漏了什么？

​

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Answer 1

实际上，当对象为null并调用其参数之一时，就会发生NPE。 这里，您的object is null。因此，当您想要调用方法create(props)时，它得到的是NPE。

我试过复制你的NPE。但我的案子没有问题。我已经在Github中发布了我的代码。你可以下载并试一试。

注:我只用过一罐log4j-1.2.17.jar

Answer 2

在rg.ops4j.pax.logger中有一个bug

log4j不会在第三点之后解析

log4j.appender.sift.appender.layout

所以我用不同的方式实现了

log4j.appender.sift.appender=\
   appender:org.apache.log4j.FileAppender, \
   appender.File:/var/log/id-@{key}.log, \
   layout:org.apache.log4j.PatternLayout, \
   layout.ConversionPattern:%d{ISO8601} | %-5.5p | %-16.16t | %-32.32c{1} | %m%n


public class MySiftAppender extends AppenderSkeleton
{
    private String key;
    private String defaultValue = "default";
    private String appender;
    private final Map<String, FileAppender> appenders = new HashMap<>();


    public String getKey()
    {
        return key;
    }

    public void setKey(String key)
    {
        this.key = key;
    }

    public String getDefault()
    {
        return defaultValue;
    }

    public void setDefault(String defaultValue)
    {
        this.defaultValue = defaultValue;
    }

    public String getAppender()
    {
        return appender;
    }

    public void setAppender(String appender)
    {
        this.appender = appender;
    }


    private FileAppender getAppender(String name) {
        FileAppender app = appenders.get(name);
        if ( app != null ) {
            return app;
        }

        synchronized ( appenders ) {
            app = appenders.get(name);
            if ( app != null ) {
                return app;
            }

            PatternLayout layout = null;

            for ( String p : appender.split("\\s*,\\s*") ) {
                String[] d = p.split("\\s*:\\s*");

                if ( d[0].equals("appender") ) {
                    app = (FileAppender) OptionConverter.instantiateByClassName(d[1], Appender.class, null);
                    app.setName(name);
                }

                if ( d[0].equals("appender.File") ) {
                    String pathname = d[1].replaceAll("\\@\\{key\\}", name);
                    app.setFile(pathname);
                }

                if ( d[0].equals("layout")) {
                    layout = (PatternLayout) OptionConverter.instantiateByClassName(d[1], Layout.class, null);
                    app.setLayout(layout);
                }

                if ( d[0].equals("layout.ConversionPattern")) {
                    layout.setConversionPattern(d[1]);
                }
            }

            app.activateOptions();

            appenders.put(name, app);

            return app;
        }
    }