更改Java类加载器的优先级(Changing the Priority of Java Classloaders)

 这可能已经被问过，但如果是这样的话，我一直都找不到它。 
 
 问 
 
 Java 1.7 / 1.8中是否存在一种方法，缺少实现自定义类加载器，以使应用程序类路径比扩展类路径具有更高优先级（先前加载）？ 
 
 问题 
 
 我们有一个使用Apache的log4j库的多个应用程序的平台。 我们还在[jre] / lib / ext中安装了一个自定义JCA安全提供程序，它也使用log4j。 为此，必须将log4j与提供程序一起安装在ext目录中。 
 
 平台上的一个应用程序（Apache的activemq）依赖于比提供者更老的log4j / slf4j版本。 由于提供程序的log4j jar位于[jre] / lib / ext中，它们会覆盖activemq，导致发生NoSuchMethodError： 
 
java.lang.NoSuchMethodError: org.slf4j.spi.LocationAwareLogger.log(Lorg/slf4j/Marker;Ljava/lang/String;ILjava/lang/String;Ljava/lang/Throwable;)V
 
 那么有没有办法让应用程序类路径中的jar取代扩展目录？ 
 
 再生产 
 
 您可以通过将这些jar安装到[jre] / lib / ext中来重现此问题： 
 
 
 
 log4j的-1.2.17.jar 
 
 
 SLF4J-API-1.7.2.jar 
 
 
 SLF4J-log4j12-1.7.2.jar 
 
 
 并通过在应用程序类路径中安装这些jar： 
 
 
 
 log4j的-1.2.14.jar 
 
 
 SLF4J-API 1.5.11.jar 
 
 
 SLF4J-log4j12-1.5.11.jar 
 
 
 并通过运行此代码： 
 
import java.security.NoSuchAlgorithmException;
import java.security.SecureRandom;

import org.slf4j.LoggerFactory;
import org.slf4j.spi.LocationAwareLogger;

public class LogApp {

    public final static LocationAwareLogger logger =
            (LocationAwareLogger) LoggerFactory.getLogger(LogApp.class);

    public static void main(String[] args) throws NoSuchAlgorithmException {
        // Make sure custom security provider has been initialized.
        SecureRandom rand = SecureRandom.getInstanceStrong();
        rand.doubles();

        //ClassLoader cl = ClassLoader.getSystemClassLoader();

        LogApp.logger.error("error: {}: {}", "string", new Exception());
        LogApp.logger.log(null, LogApp.class.getCanonicalName(),
                logger.ERROR_INT, "some message", new Exception());
    }

}
 
 哪个应该生成此错误： 
 
Exception in thread "main" java.lang.NoSuchMethodError: org.slf4j.spi.LocationAwareLogger.log(Lorg/slf4j/Marker;Ljava/lang/String;ILjava/lang/String;Ljava/lang/Throwable;)V
    at LogApp.main(LogApp.java:23)
 
 回答 
 
 使用此参数运行Java： 
 
-Xbootclasspath/p:[path_to_app_libs]/log4j-1.2.14.jar:[path_to_app_libs]/slf4j-api-1.5.11.jar:[path_to_app_libs]/slf4j-log4j12-1.5.11.jar

It's possible this has already been asked, but if so I've been unable to find it.
 
The Ask
 
Is there a way in Java 1.7/1.8, short of implementing a custom classloader, to make the application classpath higher-priority (loaded earlier) than the extension classpath?
 
The Issue
 
We have a platform with multiple apps using Apache's log4j library. We also have a custom JCA security provider installed in [jre]/lib/ext which also uses log4j. In order to do so, log4j has to be installed along with the provider in the ext directory.
 
One of the apps on the platform (Apache's activemq) relies on an older version of log4j/slf4j than the provider's. And since the provider's log4j jars are in [jre]/lib/ext, they override activemq's, causing a NoSuchMethodError to occur:
 
java.lang.NoSuchMethodError: org.slf4j.spi.LocationAwareLogger.log(Lorg/slf4j/Marker;Ljava/lang/String;ILjava/lang/String;Ljava/lang/Throwable;)V
 
So is there a way to make the jars in the application classpath supersede the extension directory?
 
Reproduction
 
You can reproduce this issue by installing these jars into [jre]/lib/ext:
 
 
 
log4j-1.2.17.jar
 
 
slf4j-api-1.7.2.jar
 
 
slf4j-log4j12-1.7.2.jar
 
 
And by installing these jars in the application classpath:
 
 
 
log4j-1.2.14.jar
 
 
slf4j-api-1.5.11.jar
 
 
slf4j-log4j12-1.5.11.jar
 
 
And by running this code:
 
import java.security.NoSuchAlgorithmException;
import java.security.SecureRandom;

import org.slf4j.LoggerFactory;
import org.slf4j.spi.LocationAwareLogger;

public class LogApp {

    public final static LocationAwareLogger logger =
            (LocationAwareLogger) LoggerFactory.getLogger(LogApp.class);

    public static void main(String[] args) throws NoSuchAlgorithmException {
        // Make sure custom security provider has been initialized.
        SecureRandom rand = SecureRandom.getInstanceStrong();
        rand.doubles();

        //ClassLoader cl = ClassLoader.getSystemClassLoader();

        LogApp.logger.error("error: {}: {}", "string", new Exception());
        LogApp.logger.log(null, LogApp.class.getCanonicalName(),
                logger.ERROR_INT, "some message", new Exception());
    }

}
 
Which should generate this error:
 
Exception in thread "main" java.lang.NoSuchMethodError: org.slf4j.spi.LocationAwareLogger.log(Lorg/slf4j/Marker;Ljava/lang/String;ILjava/lang/String;Ljava/lang/Throwable;)V
    at LogApp.main(LogApp.java:23)
 
Answer
 
Run Java with this parameter:
 
-Xbootclasspath/p:[path_to_app_libs]/log4j-1.2.14.jar:[path_to_app_libs]/slf4j-api-1.5.11.jar:[path_to_app_libs]/slf4j-log4j12-1.5.11.jar

原文：https://stackoverflow.com/questions/37188353