Generate Signed APK错误之Unable to pre-dex

今儿项目终于要上线啦，所以要编译个带签名的版本，之前调试什么的都没有问题，
结果到编译带签名的release版本时出错了。错误如下：

Error:trouble processing "javax/xml/XMLConstants.class":
Error:Ill-advised or mistaken usage of a core class (java.* or javax.*)
Error:when not building a core library.
Error:This is often due to inadvertently including a core library file
Error:in your application's project, when using an IDE (such as
Error:Eclipse). If you are sure you're not intentionally defining a
Error:core class, then this is the most likely explanation of what's
Error:going on.
Error:However, you might actually be trying to define a class in a core
Error:namespace, the source of which you may have taken, for example,
Error:from a non-Android virtual machine project. This will most
Error:assuredly not work. At a minimum, it jeopardizes the
Error:compatibility of your app with future versions of the platform.
Error:It is also often of questionable legality.
Error:If you really intend to build a core library -- which is only
Error:appropriate as part of creating a full virtual machine
Error:distribution, as opposed to compiling an application -- then use
Error:the "--core-library" option to suppress this error message.
Error:If you go ahead and use "--core-library" but are in fact
Error:building an application, then be forewarned that your application
Error:will still fail to build or run, at some point. Please be
Error:prepared for angry customers who find, for example, that your
Error:application ceases to function once they upgrade their operating
Error:system. You will be to blame for this problem.
Error:If you are legitimately using some code that happens to be in a
Error:core package, then the easiest safe alternative you have is to
Error:repackage that code. That is, move the classes in question into
Error:your own package namespace. This means that they will never be in
Error:conflict with core system classes. JarJar is a tool that may help
Error:you in this endeavor. If you find that you cannot do this, then
Error:that is an indication that the path you are on will ultimately
Error:lead to pain, suffering, grief, and lamentation.
Error:1 error; aborting
Error:Execution failed for task ':app:transformClassesWithPreDexForRelease'.
> com.android.build.api.transform.TransformException: java.lang.RuntimeException: java.lang.RuntimeException: Unable to pre-dex 'C:\Users\xxx.gradle\caches\modules-2\files-2.1\stax\stax-api\1.0.1\49c100caf72d658aca8e58bd74a4ba90fa2b0d70\stax-api-1.0.1.jar' to 'D:\project\Android\src\app\build\intermediates\transforms\preDex\release\52.jar'

查stackoverflow了解到因为引用了'com.squareup.retrofit2:converter-simplexml'需要在编译的它的时候，在build.gradle中添加

"{
exclude module: 'stax'
exclude module: 'stax-api'
exclude module: 'xpp3'
}"即
implementation ('com.squareup.retrofit2:converter-simplexml:2.3.0') {
exclude module: 'stax'
exclude module: 'stax-api'
exclude module: 'xpp3'
}

之后Generate Signed APK Build成功。
至于原理就不太清楚了。希望有知道的能给我些提示。谢啦。