基于Struts2的Java17应用升级log4j-api至2.20.0遇初始化错误求助

基于struts2-core:6.3.0.2开发的Java 17.0.5应用，依赖log4j-api:2.17.2时运行正常。将log4j-api升级至Struts官方支持的2.20.0版本后，应用启动触发初始化异常，添加log4j-core:2.20.0至类路径后问题仍未解决。

异常信息如下：

SEVERE [main] org.apache.catalina.core.StandardContext.filterStart Exception starting filter [Struts2Filter]
        java.lang.NoClassDefFoundError: Could not initialize class org.apache.logging.log4j.util.PropertiesUtil
                at org.apache.logging.log4j.status.StatusLogger.<clinit>(StatusLogger.java:77)
                at org.apache.logging.log4j.LogManager.<clinit>(LogManager.java:61)
                at org.apache.struts2.dispatcher.filter.StrutsPrepareAndExecuteFilter.<clinit>(StrutsPrepareAndExecuteFilter.java:49)
                at java.base/jdk.internal.reflect.NativeConstructorAccessorImpl.newInstance0(Native Method)
                at java.base/jdk.internal.reflect.NativeConstructorAccessorImpl.newInstance(NativeConstructorAccessorImpl.java:77)
                at java.base/jdk.internal.reflect.DelegatingConstructorAccessorImpl.newInstance(DelegatingConstructorAccessorImpl.java:45)
                at java.base/java.lang.reflect.Constructor.newInstanceWithCaller(Constructor.java:499)
                at java.base/java.lang.reflect.Constructor.newInstance(Constructor.java:480)
                at org.apache.catalina.core.DefaultInstanceManager.newInstance(DefaultInstanceManager.java:143)
                at org.apache.catalina.core.ApplicationFilterConfig.getFilter(ApplicationFilterConfig.java:242)
                at org.apache.catalina.core.ApplicationFilterConfig.<init>(ApplicationFilterConfig.java:97)
                at org.apache.catalina.core.StandardContext.filterStart(StandardContext.java:4311)
                at org.apache.catalina.core.StandardContext.startInternal(StandardContext.java:4940)
                at org.apache.catalina.util.LifecycleBase.start(LifecycleBase.java:171)
                at org.apache.catalina.core.ContainerBase.addChildInternal(ContainerBase.java:683)
                at org.apache.catalina.core.ContainerBase.addChild(ContainerBase.java:658)
                at org.apache.catalina.core.StandardHost.addChild(StandardHost.java:661)
                at org.apache.catalina.startup.HostConfig.deployWAR(HostConfig.java:1014)
                at org.apache.catalina.startup.HostConfig$DeployWar.run(HostConfig.java:1866)
                at java.base/java.util.concurrent.Executors$RunnableAdapter.call(Executors.java:539)
                at java.base/java.util.concurrent.FutureTask.run(FutureTask.java:264)
                at org.apache.tomcat.util.threads.InlineExecutorService.execute(InlineExecutorService.java:75)
                at java.base/java.util.concurrent.AbstractExecutorService.submit(AbstractExecutorService.java:123)
                at org.apache.catalina.startup.HostConfig.deployWARs(HostConfig.java:816)
                at org.apache.catalina.startup.HostConfig.deployApps(HostConfig.java:468)
                at org.apache.catalina.startup.HostConfig.start(HostConfig.java:1584)
                at org.apache.catalina.startup.HostConfig.lifecycleEvent(HostConfig.java:312)
                at org.apache.catalina.util.LifecycleBase.fireLifecycleEvent(LifecycleBase.java:114)
                at org.apache.catalina.util.LifecycleBase.setStateInternal(LifecycleBase.java:402)
                at org.apache.catalina.util.LifecycleBase.setState(LifecycleBase.java:345)
                at org.apache.catalina.core.ContainerBase.startInternal(ContainerBase.java:893)
                at org.apache.catalina.core.StandardHost.startInternal(StandardHost.java:794)
                at org.apache.catalina.util.LifecycleBase.start(LifecycleBase.java:171)
                at org.apache.catalina.core.ContainerBase$StartChild.call(ContainerBase.java:1332)
                at org.apache.catalina.core.ContainerBase$StartChild.call(ContainerBase.java:1322)
                at java.base/java.util.concurrent.FutureTask.run(FutureTask.java:264)
                at org.apache.tomcat.util.threads.InlineExecutorService.execute(InlineExecutorService.java:75)
                at java.base/java.util.concurrent.AbstractExecutorService.submit(AbstractExecutorService.java:145)
                at org.apache.catalina.core.ContainerBase.startInternal(ContainerBase.java:866)
                at org.apache.catalina.core.StandardEngine.startInternal(StandardEngine.java:248)
                at org.apache.catalina.util.LifecycleBase.start(LifecycleBase.java:171)
                at org.apache.catalina.core.StandardService.startInternal(StandardService.java:433)
                at org.apache.catalina.util.LifecycleBase.start(LifecycleBase.java:171)
                at org.apache.catalina.core.StandardServer.startInternal(StandardServer.java:925)
                at org.apache.catalina.util.LifecycleBase.start(LifecycleBase.java:171)
                at org.apache.catalina.startup.Catalina.start(Catalina.java:735)
                at java.base/jdk.internal.reflect.NativeMethodAccessorImpl.invoke0(Native Method)
                at java.base/jdk.internal.reflect.NativeMethodAccessorImpl.invoke(NativeMethodAccessorImpl.java:77)
                at java.base/jdk.internal.reflect.DelegatingMethodAccessorImpl.invoke(DelegatingMethodAccessorImpl.java:43)
                at java.base/java.lang.reflect.Method.invoke(Method.java:568)
                at org.apache.catalina.startup.Bootstrap.start(Bootstrap.java:345)
                at org.apache.catalina.startup.Bootstrap.main(Bootstrap.java:473)
        


Caused by: java.lang.ExceptionInInitializerError: Exception java.lang.IllegalArgumentException: key can't be empty [in thread "main"]
                at java.base/java.lang.System.checkKey(System.java:1049)
                at java.base/java.lang.System.getProperty(System.java:912)
                at org.apache.logging.log4j.util.SystemPropertiesPropertySource.getProperty(SystemPropertiesPropertySource.java:96)
                at org.apache.logging.log4j.util.SystemPropertiesPropertySource.containsProperty(SystemPropertiesPropertySource.java:104)
                at org.apache.logging.log4j.util.PropertiesUtil$Environment.lambda$null$2(PropertiesUtil.java:498)
                at java.base/java.lang.Iterable.forEach(Iterable.java:75)
                at org.apache.logging.log4j.util.PropertiesUtil$Environment.lambda$reload$3(PropertiesUtil.java:497)
                at java.base/java.util.stream.ForEachOps$ForEachOp$OfRef.accept(ForEachOps.java:183)
                at java.base/java.util.stream.ReferencePipeline$2$1.accept(ReferencePipeline.java:179)
                at java.base/java.util.HashMap$KeySpliterator.forEachRemaining(HashMap.java:1707)
                at java.base/java.util.stream.AbstractPipeline.copyInto(AbstractPipeline.java:509)
                at java.base/java.util.stream.AbstractPipeline.wrapAndCopyInto(AbstractPipeline.java:499)
                at java.base/java.util.stream.ForEachOps$ForEachOp.evaluateSequential(ForEachOps.java:150)
                at java.base/java.util.stream.ForEachOps$ForEachOp$OfRef.evaluateSequential(ForEachOps.java:173)
                at java.base/java.util.stream.AbstractPipeline.evaluate(AbstractPipeline.java:234)
                at java.base/java.util.stream.ReferencePipeline.forEach(ReferencePipeline.java:596)
                at org.apache.logging.log4j.util.PropertiesUtil$Environment.reload(PropertiesUtil.java:494)
                at org.apache.logging.log4j.util.PropertiesUtil$Environment.<init>(PropertiesUtil.java:469)
                at org.apache.logging.log4j.util.PropertiesUtil$Environment.<init>(PropertiesUtil.java:444)
                at org.apache.logging.log4j.util.PropertiesUtil.<init>(PropertiesUtil.java:88)
                at org.apache.logging.log4j.util.PropertiesUtil.<init>(PropertiesUtil.java:80)
                at org.apache.logging.log4j.util.PropertiesUtil.<clinit>(PropertiesUtil.java:56)
                at org.apache.logging.log4j.util.Constants.<clinit>(Constants.java:30)
                at org.apache.logging.log4j.spi.AbstractLogger.createClassForProperty(AbstractLogger.java:203)
                at org.apache.logging.log4j.spi.AbstractLogger.<clinit>(AbstractLogger.java:92)
                ... 51 more

从异常根因java.lang.IllegalArgumentException: key can't be empty可以看出，Log4j 2.20.0在初始化PropertiesUtil时会严格校验系统属性的键合法性，而你的运行环境中存在空键的系统属性（比如错误的JVM启动参数-D=、空名称的环境变量），旧版本2.17.2没有该校验逻辑，因此之前运行正常。

按以下步骤排查解决：

• 检查JVM启动参数：查看IDE的VM选项、Tomcat启动脚本（catalina.sh/catalina.bat），删除类似-D=这类无效的空键参数。
• 清理环境变量：排查系统或用户环境变量中是否存在空名称的变量，这类变量会被自动转为系统属性，导致空键问题。
• 校验依赖一致性：确保log4j-api和log4j-core版本完全一致（均为2.20.0），用依赖分析工具（Maven执行mvn dependency:tree，Gradle执行gradle dependencies）排查是否有旧版本Log4j类残留，排除冲突依赖。
• 检查Tomcat配置：如果部署在Tomcat中，确认setenv.sh/setenv.bat里没有错误的环境变量配置，避免生成空键系统属性。

内容的提问来源于stack exchange，提问作者kritarth