I was trying to run a spring-boot application which uses an Orika mapper from one of it’s dependencies (the dependency is released as a jar):

Caused by: ma.glasnost.orika.MappingException: java.lang.ClassCastException: ma.glasnost.orika.generated.Orika_UserDest_UserSource_Mapper3053159455512$0 cannot be cast to ma.glasnost.orika.impl.GeneratedObjectBase
	at ma.glasnost.orika.impl.generator.MapperGenerator.build(MapperGenerator.java:104) ~[orika-core-1.5.1.jar:na]
	at ma.glasnost.orika.impl.DefaultMapperFactory.buildMapper(DefaultMapperFactory.java:1480) ~[orika-core-1.5.1.jar:na]
	at ma.glasnost.orika.impl.DefaultMapperFactory.build(DefaultMapperFactory.java:1295) ~[orika-core-1.5.1.jar:na]
	at ma.glasnost.orika.impl.DefaultMapperFactory.getMapperFacade(DefaultMapperFactory.java:883) ~[orika-core-1.5.1.jar:na]
	at ma.glasnost.orika.impl.ConfigurableMapper.init(ConfigurableMapper.java:121) ~[orika-core-1.5.1.jar:na]
	at ma.glasnost.orika.impl.ConfigurableMapper.<init>(ConfigurableMapper.java:97) ~[orika-core-1.5.1.jar:na]
	at com.liguoliang.common.mapper.UserMapper.<init>(UserMapper.java:8) ~[common-1.0-SNAPSHOT.jar:na]
	at com.liguoliang.springboot1.helloworld.TestApp.run(TestApp.java:30) ~[classes/:na]
	at org.springframework.boot.SpringApplication.callRunner(SpringApplication.java:732) [spring-boot-1.5.15.RELEASE.jar:1.5.15.RELEASE]
	... 11 common frames omitted
Caused by: java.lang.ClassCastException: ma.glasnost.orika.generated.Orika_UserDest_UserSource_Mapper3053159455512$0 cannot be cast to ma.glasnost.orika.impl.GeneratedObjectBase
	at ma.glasnost.orika.impl.generator.SourceCodeContext.getInstance(SourceCodeContext.java:262) ~[orika-core-1.5.1.jar:na]
	at ma.glasnost.orika.impl.generator.MapperGenerator.build(MapperGenerator.java:73) ~[orika-core-1.5.1.jar:na]
	... 19 common frames omitted

I was confused by this error because the class Orika_UserDest_UserSource_Mapper3053159455512$0 obviously is a generated class and it supposed to work with Orika itself.

to illustrate the project structure:

the spring-boot I’m currently working on:

<project>
	<groupId>com.liguoliang</groupId>
	<artifactId>spring-boot-1.5-hello-world</artifactId>
	<version>0.0.1-SNAPSHOT</version>
	<packaging>jar</packaging>

	<parent>
		<groupId>org.springframework.boot</groupId>
		<artifactId>spring-boot-starter-parent</artifactId>
		<version>1.5.15.RELEASE</version>
		<relativePath/> <!-- lookup parent from repository -->
	</parent>

	<dependencies>
        <dependency>
            <groupId>org.springframework.boot</groupId>
            <artifactId>spring-boot-starter</artifactId>
        </dependency>
		<dependency>
			<groupId>com.liguoliang</groupId>
			<artifactId>common-demo</artifactId>
			<version>0.0.1</version>
		</dependency>
        <dependency>
            <groupId>org.springframework.boot</groupId>
            <artifactId>spring-boot-devtools</artifactId>
            <optional>true</optional>
        </dependency>
    </dependencies>
 </project>

the dependency jar file’s pom: (which is where the mapper come form):

<?xml version="1.0" encoding="UTF-8"?>
<project xmlns="http://maven.apache.org/POM/4.0.0"
         xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance"
         xsi:schemaLocation="http://maven.apache.org/POM/4.0.0 http://maven.apache.org/xsd/maven-4.0.0.xsd">
    <modelVersion>4.0.0</modelVersion>

    <groupId>com.liguoliang</groupId>
    <artifactId>common-demo</artifactId>
    <version>0.0.1</version>

    <packaging>jar</packaging>

    <dependencies>
        <dependency>
            <groupId>net.rakugakibox.spring.boot</groupId>
            <artifactId>orika-spring-boot-starter</artifactId>
            <version>1.5.0</version>
        </dependency>
    </dependencies>
</project>

What I found by debugging

the issue happens here:

// SourceCodeContext.java
    public <T extends GeneratedObjectBase> T getInstance() throws SourceCodeGenerationException, InstantiationException,
            IllegalAccessException {
        T instance = (T) compileClass().newInstance();
        ...
        }

the compileClass() shows that the source of the generated class is:

package ma.glasnost.orika.generated;
public class Orika_UserDest_UserSource_Mapper3053159455512$0 extends ma.glasnost.orika.impl.GeneratedMapperBase {
...

GeneratedMapperBase extends GeneratedObjectBase, by reading the code, I’m sure that compileClass().newInstance() must be a GeneratedObjectBase .

but issue happened, consistently. in the runtime, instance a of A is not a A, that could be caused by A was loaded in multiple loader? I found:

  • GeneratedClass.class was loaded by Launcher$AppClassLoader
  • compileClass() and compileClass().getSuperclass().getSuperclass()(which is GeneratedObjectBase) were loaded by RestartClassLoader, RestartClassLoader.parent is the Launcher$AppClassLoader

so the error message can be enhanced to show that: jvm failed to cast instance of GeneratedObjectBase loaded by RestartClassLoader to GeneratedObjectBase loaded by AppClassLoader

Why GeneratedObjectBase got loaded by two different class loaders?

  • before the mapper get actually initialized, the mapper(part of the common-demo lib) get loaded by AppClassLoader, probably together with the mapper, GeneratedClassBase also get loaded by AppClassLoader
  • when the mapper getting initialized, spring-boot devtools will use the RestartClassLoader to load classes in the workspace of the IDE, to get faster ‘reload’:
    • Orika used the RestartClassLoader to compile the generated class: 
       // JavassistCompilerStrategy.compileClass()
  compiledClass = byteCodeClass.toClass(Thread.currentThread().getContextClassLoader(), this.getClass().getProtectionDomain());

      so the generated class and the parent classes all get loaded by RestartClassLoader that how did the exception happen.

How to fix?

Conclusion

Restart functionality from Spring-boot devtools can be very helpful when working in an IDE, the application get restarted automatically on class change. however issues can happen when class loaders get messed up. e.g. due to the dependency structure, GeneratedObjectBase was loaded by two loaders which caused the exception.