JHades - Your way out of Jar Hell!
JHades is a Java classpath troubleshooting tool. It helps to solve common Java classpath / Jar Hell related problems such as ClassNotFoundException, NoClassDefFoundError, ClassCastException, NoSuchMethodException, LinkageError, etc.
It allows to query the classpath for class duplicates, find specific resource locations, etc.
Documentation available at http://jhades.org
Example of a classpath problem
Let's say a java web application works fine in development, but when deploying it to a server the following problem occurs:
java.lang.ClassCastException: org.apache.bval.jsr303.ApacheValidationProvider cannot be cast to javax.validation.spi.ValidationProvider javax.validation.Validation$DefaultValidationProviderResolver.getValidationProviders(Validation.java:332) javax.validation.Validation$GenericBootstrapImpl.configure(Validation.java:256) javax.validation.Validation.buildDefaultValidatorFactory(Validation.java:111) classloaders.test.TestServlet.doGet(TestServlet.java:33) javax.servlet.http.HttpServlet.service(HttpServlet.java:621) javax.servlet.http.HttpServlet.service(HttpServlet.java:728) org.apache.tomcat.websocket.server.WsFilter.doFilter(WsFilter.java:91)
How to troubleshoot
JHades provides several commands that allow to extract from the runtime environment a lot of information which is important for understanding what is going on. Namelly these types of questions can be answered:
- are there any overlapping jars, with different versions of the same class?
- Are those multiple versions of the same class all identical or not ?
- where are the different versions of a given class located ?
- which class loader is loading which version of a given class ?
- What does the chain of classloaders look like, how are the classloaders configured ?
This is an example of how to answer several of these questions for the sample problem above:
new JHades() .printClassLoaders() .printClasspath() .overlappingJarsReport() .multipleClassVersionsReport() .findClassByName("org.apache.bval.jsr303.ApacheValidationProvider") .findClassByName("javax.validation.spi.ValidationProvider");
Jar overlap report
One of the most common sources of classpath problems are overlapping jar files, that contain multiple versions of the same class:
>>>> Jar overlap report: aspectjrt-1.7.2.jar overlaps with aspectjweaver-1.7.2.jar - total overlapping classes: 129 ejb3-persistence-1.0.2.GA.jar overlaps with hibernate-jpa-2.0-api-1.0.1.Final.jar - total overlapping classes: 91 stax-api-1.0-2.jar overlaps with xml-apis-1.4.01.jar - total overlapping classes: 34 javax.xml.soap-api-1.3.5.jar overlaps with saaj-api-1.3.jar - total overlapping classes: 29 commons-beanutils-1.8.3.jar overlaps with commons-collections-3.2.1.jar - total overlapping classes: 10 xbean-2.2.0.jar overlaps with xml-apis-1.4.01.jar - total overlapping classes: 6 commons-logging-1.1.1.jar overlaps with jcl-over-slf4j-1.6.6.jar - total overlapping classes: 6 Total number of classes with more than one version: 305
Check http://jhades.org for more details.