Java如何选择.JAR库版本

Question

For various historical reasons, I have an old java application which shares a local lib/ directory of library .JARs with some other applications. In this directory is a number of versioned copies of the same library, for example:

...
log4j-1.2.16.jar
log4j-1.2.17.jar
slf4j-api-1.7.5.jar
slf4j-api-1.7.21.jar
slf4j-log4j12-1.7.5.jar
slf4j-log4j12-1.7.9.jar
...

When my java app starts, how does it decide which .jar file to load? As far as I know, the CLASSPATH is just set to ./lib/ . But it uses the Tanuki Service wrapper to start, so I'm not 100% sure of this.

I can't delete any of these existing .JARs, is there a way to specify exactly which .JARs my app will use ?

After this old application was patched, now I'm getting a NoClassDefFoundError, and I suspect the app is loading an older (or newer) version of a .JAR that conflicts with another library (BoneCP and slf4j).

Answer 1

Which jar first in classpath that is picked up when JVM classloader is looking for a class. So you can try to add those jars in different order to check which one breaks your application

Answer 2

If you are going to share lib directories like that, then each application needs to list the JAR files that it uses explicitly on the classpath. If you use a wildcard classpath entry (eg "lib/*"), then it is not specified which versions of the JARs will be used. The manual entry states:

Class Path Wild Cards

Class path entries can contain the base name wildcard character ( ), which is considered equivalent to specifying a list of all of the files in the directory with the extension .jar or .JAR. For example, the class path entry mydir/ specifies all JAR files in the directory named mydir. A class path entry consisting of * expands to a list of all the jar files in the current directory. Files are considered regardless of whether they are hidden (have names beginning with '.').

....

The order in which the JAR files in a directory are enumerated in the expanded class path is not specified and may vary from platform to platform and even from moment to moment on the same machine. A well-constructed application should not depend upon any particular order. If a specific order is required, then the JAR files can be enumerated explicitly in the class path.