GitHub user zhanjinhao created a discussion: Using Custom ClassLoader to
Isolate Log42j and Slf4j. Console log: Properties contains an invalid element
or attribute "property"
env:
1. log4j2 version: 2.17.2
2. slf4j version: 1.7.36
3. jdk version: 8
4. os version: win11
### Requirements
I want to develop an Agent to trace service status. To isolate Log42j and
Slf4j, I develop a custom classloader which named LogClassLoader.
The main requirements are:
1. Load jar from specific directory, not the AppClassLoader's classpath
5. Load log4j2.xml from specific direcotry
6. output the msg to the path defined in log4j2.xml.
### Code and Problem
LogClassLoader override loadClass() method. When it works, if the name has
prefix about Log4j2 or Slf4j, I will load class from the specific directory.
the code about LogClassLoader is copy from apache skywalking AgentClassLoader.
The code of LogClassLoader is:
```
/**
* 用于加载插件和插件的拦截器
*/
public class LogClassLoader extends ClassLoader {
/**
* 用于加载日志组件的加载器
*/
@Getter
private static LogClassLoader DEFAULT_LOADER;
/**
* 自定义类加载器加载类的路径
*/
private File classpath;
private List<Jar> allJarList;
private ReentrantLock jarScanLock = new ReentrantLock();
public LogClassLoader(ClassLoader parent) {
super(parent);
File agentJarDir = AgentPackagePath.getPath();
System.out.println(String.format("logLibDir: %s", agentJarDir));
classpath = new File(new File(agentJarDir, "lib"), "log");
getAllJarList();
}
public static void initDefaultLoader() {
if (DEFAULT_LOADER == null) {
DEFAULT_LOADER = new
LogClassLoader(LogClassLoader.class.getClassLoader());
}
}
private static List<String> logPrefixList = new ArrayList<>();
static {
logPrefixList.add("org.slf4j.");
logPrefixList.add("org.apache.logging.log4j.");
logPrefixList.add("org.apache.logging.slf4j.");
}
@Override
public Class<?> loadClass(String name, boolean resolve) throws
ClassNotFoundException {
for (String logPrefix : logPrefixList) {
if (name.startsWith(logPrefix)) {
Class<?> aClass = doFindClass(name);
if (resolve) {
resolveClass(aClass);
}
}
}
return super.loadClass(name, resolve);
}
/**
* loadClass --> findClass（自定义自己的类加载逻辑） --> defineClass
*/
@Override
protected Class<?> findClass(String name) throws ClassNotFoundException {
return super.findClass(name);
}
private Class<?> doFindClass(String name) throws ClassNotFoundException {
List<Jar> _allJarList = getAllJarList();
String concat = name.replace(".", "/").concat(".class");
for (Jar jar : _allJarList) {
JarEntry jarEntry = jar.jarFile.getJarEntry(concat);
if (jarEntry == null) {
continue;
}
try {
URL url = new URL("jar:file:" + jar.sourceFile.getAbsolutePath() + "!/"
+ concat);
byte[] byteArray = IOUtils.toByteArray(url);
return defineClass(name, byteArray, 0, byteArray.length);
} catch (Exception e) {
System.out.println(String.format("find class %s error", name));
}
}
throw new ClassNotFoundException("can not find " + name);
}
private List<Jar> getAllJarList() {
if (allJarList == null) {
jarScanLock.lock();
try {
if (allJarList == null) {
allJarList = doGetJars();
}
} finally {
jarScanLock.unlock();
}
}
return allJarList;
}
private List<Jar> doGetJars() {
List<Jar> _allJarList = new ArrayList<>();
if (classpath.exists() && classpath.isDirectory()) {
String[] list = classpath.list(new FilenameFilter() {
@Override
public boolean accept(File dir, String name) {
return name.endsWith(".jar");
}
});
if (list == null || list.length == 0) {
return _allJarList;
}
for (String s : list) {
File jarSourceFile = new File(classpath, s);
try {
Jar jar = new Jar(new JarFile(jarSourceFile), jarSourceFile);
_allJarList.add(jar);
System.out.println(String.format("load jar %s success.",
jarSourceFile));
} catch (Exception e) {
System.out.println(String.format("jar %s load fail.", jarSourceFile));
e.printStackTrace();
}
}
}
return _allJarList;
}
private static class Jar {
/**
* jar文件对对应的jarFile对象
*/
private final JarFile jarFile;
/**
* jar文件
*/
private final File sourceFile;
public Jar(JarFile jarFile, File sourceFile) {
this.jarFile = jarFile;
this.sourceFile = sourceFile;
}
}
}
```
My log4j2.xml is :
```
<?xml version="1.0" encoding="UTF-8"?>
<configuration monitorInterval="5">
<!--变量配置-->
<Properties>
<!-- 格式化输出：%date表示日期，%thread表示线程名，%-5level：级别从左显示5个字符宽度 %msg：日志消息，%n是换行符-->
<!-- %logger{36} 表示 Logger 名字最长36个字符 -->
<property name="LOG_PATTERN">external3333 - %date{HH:mm:ss.SSS} [%thread]
%-5level %logger{36} - %msg%n</property>
<!-- 定义日志存储的路径 -->
<property name="FILE_PATH">./agent_logs</property>
<property name="FILE_NAME">agentLogs</property>
</Properties>
<Appenders>
<Console name="Console" target="SYSTEM_OUT">
<!--设置日志格式及颜色-->
<PatternLayout
pattern="external3333 - %date{HH:mm:ss.SSS} [%thread] %-5level
%logger{36} - %msg%n"
disableAnsi="false" noConsoleNoAnsi="false"/>
</Console>
<!--文件会打印出所有信息，这个log每次运行程序会自动清空，由append属性决定，适合临时测试用-->
<File name="Filelog" fileName="${FILE_PATH}/test.log" append="false">
<PatternLayout pattern="${LOG_PATTERN}"/>
</File>
<!--
这个会打印出所有的info及以下级别的信息，每次大小超过size，则这size大小的日志会自动存入按年份-月份建立的文件夹下面并进行压缩，作为存档-->
<RollingFile name="RollingFileInfo" fileName="${FILE_PATH}/info.log"
filePattern="${FILE_PATH}/${FILE_NAME}-INFO-%d{yyyy-MM-dd}_%i.log.gz">
<!--控制台只输出level及以上级别的信息（onMatch），其他的直接拒绝（onMismatch）-->
<ThresholdFilter level="info" onMatch="ACCEPT" onMismatch="DENY"/>
<PatternLayout pattern="${LOG_PATTERN}"/>
<Policies>
<!--interval属性用来指定多久滚动一次，默认是1 hour-->
<TimeBasedTriggeringPolicy interval="1"/>
<SizeBasedTriggeringPolicy size="10MB"/>
</Policies>
<!-- DefaultRolloverStrategy属性如不设置，则默认为最多同一文件夹下7个文件开始覆盖-->
<DefaultRolloverStrategy max="15"/>
</RollingFile>
<!--
这个会打印出所有的warn及以下级别的信息，每次大小超过size，则这size大小的日志会自动存入按年份-月份建立的文件夹下面并进行压缩，作为存档-->
<RollingFile name="RollingFileWarn" fileName="${FILE_PATH}/warn.log"
filePattern="${FILE_PATH}/${FILE_NAME}-WARN-%d{yyyy-MM-dd}_%i.log.gz">
<!--控制台只输出level及以上级别的信息（onMatch），其他的直接拒绝（onMismatch）-->
<ThresholdFilter level="warn" onMatch="ACCEPT" onMismatch="DENY"/>
<PatternLayout pattern="${LOG_PATTERN}"/>
<Policies>
<!--interval属性用来指定多久滚动一次，默认是1 hour-->
<TimeBasedTriggeringPolicy interval="1"/>
<SizeBasedTriggeringPolicy size="10MB"/>
</Policies>
<!-- DefaultRolloverStrategy属性如不设置，则默认为最多同一文件夹下7个文件开始覆盖-->
<DefaultRolloverStrategy max="15"/>
</RollingFile>
<!--
这个会打印出所有的error及以下级别的信息，每次大小超过size，则这size大小的日志会自动存入按年份-月份建立的文件夹下面并进行压缩，作为存档-->
<RollingFile name="RollingFileError" fileName="${FILE_PATH}/error.log"
filePattern="${FILE_PATH}/${FILE_NAME}-ERROR-%d{yyyy-MM-dd}_%i.log.gz">
<!--控制台只输出level及以上级别的信息（onMatch），其他的直接拒绝（onMismatch）-->
<ThresholdFilter level="error" onMatch="ACCEPT" onMismatch="DENY"/>
<PatternLayout pattern="${LOG_PATTERN}"/>
<Policies>
<!--interval属性用来指定多久滚动一次，默认是1 hour-->
<TimeBasedTriggeringPolicy interval="1"/>
<SizeBasedTriggeringPolicy size="10MB"/>
</Policies>
<!-- DefaultRolloverStrategy属性如不设置，则默认为最多同一文件夹下7个文件开始覆盖-->
<DefaultRolloverStrategy max="15"/>
</RollingFile>
</Appenders>
<!--Logger节点用来单独指定日志的形式，比如要为指定包下的class指定不同的日志级别等。-->
<!--然后定义loggers，只有定义了logger并引入的appender，appender才会生效-->
<loggers>
<root level="info">
<appender-ref ref="Console"/>
<appender-ref ref="Filelog"/>
<appender-ref ref="RollingFileInfo"/>
<appender-ref ref="RollingFileWarn"/>
<appender-ref ref="RollingFileError"/>
</root>
</loggers>
</configuration>
```
In order to load jar and log4j2.xml from specific directory, I use
Class.forName(className, true, LogClassLoader) to load class. And I use
reflection api to invoke getContext(ClassLoader.class, boolean.class,
URI.class) method to load log4j2.xml, which avoid use the configuration in
AppClassLoader's classpath. And the code is:
```
public class MyLoggerFactory {
static Class<?> loggerFactoryClass;
// 添加Logger接口的引用
private static Class<?> loggerInterface;
static Method getLoggerString;
static Method getLoggerClass;
static Class<?> logManagerClass;
static Class<?> loggerContextClass;
static {
try {
LogClassLoader.initDefaultLoader();
LogClassLoader defaultLoader = LogClassLoader.getDEFAULT_LOADER();
ClassLoader contextClassLoader =
Thread.currentThread().getContextClassLoader();
Thread.currentThread().setContextClassLoader(defaultLoader);
try {
loggerFactoryClass = Class.forName("org.slf4j.LoggerFactory", true,
defaultLoader);
loggerInterface = Class.forName("org.slf4j.Logger", true,
defaultLoader);
logManagerClass = Class.forName("org.apache.logging.log4j.LogManager",
true, defaultLoader);
loggerContextClass =
Class.forName("org.apache.logging.log4j.core.LoggerContext", true,
defaultLoader);
getLoggerString = loggerFactoryClass.getDeclaredMethod("getLogger",
String.class);
getLoggerClass = loggerFactoryClass.getDeclaredMethod("getLogger",
Class.class);
File agentJarDir = AgentPackagePath.getPath();
File conFile = new File(new File(new File(AgentPackagePath.getPath(),
"lib"), "log"), "log4j2.xml");
// ConfigurationSource source = new ConfigurationSource(new
FileInputStream(conFile), conFile);
// Configuration configuration = new XmlConfiguration(null, source);
// Configurator.initialize(configuration);
// config dist/lib/log/log4j2.xml
Method getContextMethod =
logManagerClass.getDeclaredMethod("getContext", ClassLoader.class,
boolean.class, URI.class);
Object logContext = getContextMethod.invoke(null, null, false,
conFile.toURI());
} finally {
Thread.currentThread().setContextClassLoader(contextClassLoader);
}
} catch (Exception e) {
e.printStackTrace();
}
}
public static Logger getLogger(Class<?> clazz) {
try {
Object invoke = getLoggerClass.invoke(null, clazz);
// 使用代理模式包装返回的对象，避免类型转换问题
if (invoke != null) {
return createLoggerProxy(invoke);
}
throw new RuntimeException(String.format("Failed to create logger %s
instance", clazz));
} catch (Exception e) {
throw new RuntimeException(e);
}
}
// 创建代理Logger实例来避免类加载器问题
private static Logger createLoggerProxy(Object loggerInstance) {
// 实现代理逻辑，将方法调用转发给实际的logger实例
// 这样可以绕过类加载器类型检查问题
return (Logger) Proxy.newProxyInstance(
MyLoggerFactory.class.getClassLoader(),
new Class[]{Logger.class},
new InvocationHandler() {
@Override
public Object invoke(Object proxy, Method method, Object[] args)
throws Throwable {
// 在目标对象上查找匹配的方法
Method targetMethod = loggerInstance.getClass().getMethod(
method.getName(), method.getParameterTypes());
return targetMethod.invoke(loggerInstance, args);
}
}
);
}
}
```
When it works, I find it can load jar and log4j2.xml from the specific
directory.
**BUT, it can not output the msg to the path defined in property of
configuration!**
The result is it cannot replace the variables of Appenders , although the
variables are defined in properties element.
<img width="1899" height="805" alt="image"
src="https://github.com/user-attachments/assets/28bb8f45-2a7d-493b-b50a-dfbc0bb52bbd";
/>
The config can work as expected in springboot's project.
The full demo project is:
https://github.com/[zhanjinhao/log-learn](https://github.com/zhanjinhao/log-learn)
and it can reproduce the problem. **Run Application in springboot way, the
console will output "Properties contains an invalid element or attribute
"property"" and the file will be created in ${FILE_PATH} not in "agent_logs"**
### DEBUG
I try to debug the code to find some reason.
in org.apache.logging.log4j.core.config.AbstractConfiguration#doConfigure, the
children of Properties is empty.
<img width="2182" height="1459" alt="image"
src="https://github.com/user-attachments/assets/c2b201c3-5565-4f59-98bb-afbee984083f";
/>
In springboot project, the children is an List which contains three children.
<img width="2346" height="1405" alt="image"
src="https://github.com/user-attachments/assets/3fd81366-32d5-4fcf-9158-ca81fa4456a7";
/>
But i don't know why the children are different. I don't have knowledge about
xml parsing.
Can somebody give me some advice! Thank a lot.
GitHub link: https://github.com/apache/logging-log4j2/discussions/3960
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