从jar加载自定义类时发生ClassCastException [英] ClassCastException on custom class loading from jar
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问题描述
出于教育目的，我正在尝试实现自定义类加载器.
I'm trying to implement custom class loader for educational purposes.
我在jar文件中有模块天气"，我想由JarClassLoader从App类加载.
I have the module "Weather" in jar file which I want load from App class by JarClassLoader.
此处的类加载器(它从指定的类加载所有类罐子):
Classloader from here (it loads all classes from the specified jar):
package com.example.classloading;
import java.io.IOException;
import java.io.InputStream;
import java.util.Enumeration;
import java.util.HashMap;
import java.util.jar.JarEntry;
import java.util.jar.JarFile;
public class JarClassLoader extends ClassLoader {
private HashMap<String, Class<?>> cache = new HashMap<String, Class<?>>();
private String jarFileName;
private String packageName;
private static String WARNING = "Warning : No jar file found. Packet unmarshalling won't be possible. Please verify your classpath";
public JarClassLoader(String jarFileName, String packageName) {
this.jarFileName = jarFileName;
this.packageName = packageName;
cacheClasses();
}
private void cacheClasses() {
try {
JarFile jarFile = new JarFile(jarFileName);
Enumeration entries = jarFile.entries();
while (entries.hasMoreElements()) {
JarEntry jarEntry = (JarEntry) entries.nextElement();
// simple class validation based on package name
if (match(normalize(jarEntry.getName()), packageName)) {
byte[] classData = loadClassData(jarFile, jarEntry);
if (classData != null) {
Class<?> clazz = defineClass(stripClassName(normalize(jarEntry.getName())), classData, 0, classData.length);
cache.put(clazz.getName(), clazz);
System.out.println("== class " + clazz.getName() + " loaded in cache");
}
}
}
}
catch (IOException IOE) {
System.out.println(WARNING);
}
}
public synchronized Class<?> loadClass(String name) throws ClassNotFoundException {
Class<?> result = cache.get(name);
if (result == null)
result = cache.get(packageName + "." + name);
if (result == null)
result = super.findSystemClass(name);
System.out.println("== loadClass(" + name + ")");
return result;
}
private String stripClassName(String className) {
return className.substring(0, className.length() - 6);
}
private String normalize(String className) {
return className.replace('/', '.');
}
private boolean match(String className, String packageName) {
return className.startsWith(packageName) && className.endsWith(".class");
}
private byte[] loadClassData(JarFile jarFile, JarEntry jarEntry) throws IOException {
long size = jarEntry.getSize();
if (size == -1 || size == 0)
return null;
byte[] data = new byte[(int)size];
InputStream in = jarFile.getInputStream(jarEntry);
in.read(data);
return data;
}
}
接口和实现(只是没有任何特定逻辑的模板):
Interface and implementation (just template without any specific logic):
package com.example.classloading;
public interface Module {
public void demo(String str);
}
package com.example.classloading;
public class Weather implements Module {
public void demo(String str) {
System.out.println("hello from weather module");
}
}
应用类:
import com.example.classloading.JarClassLoader;
import com.example.classloading.Module;
public class App {
public static void main(String[] args) {
JarClassLoader jarClassLoader = new JarClassLoader("classloading/weather-module/target/weather-module-1.0-SNAPSHOT.jar", "com.example.classloading");
try {
Class<?> clas = jarClassLoader.loadClass("com.example.classloading.Weather");
Module sample = (Module) clas.newInstance();
sample.demo("1");
} catch (ClassNotFoundException e) {
e.printStackTrace();
} catch (IllegalAccessException e) {
e.printStackTrace();
} catch (InstantiationException e) {
e.printStackTrace();
}
}
}
问题:运行主方法时，得到以下输出:
== loadClass(java.lang.Object)
== class com.example.classloading.Module loaded in cache
== class com.example.classloading.Weather loaded in cache
== loadClass(com.example.classloading.Weather)
Exception in thread "main" java.lang.ClassCastException: com.example.classloading.Weather cannot be cast to com.example.classloading.Module
at App.main(App.java:12)
逻辑或语法是否存在问题? Module是不是由应用程序类加载器加载的?
Is there a problem in a logic or syntax? Module doesn't loaded by application class loader?
文件树(略有简化):
├───classloading
│ │ pom.xml
│ │
│ ├───menu-module
│ │ │ pom.xml
│ │ │
│ │ ├───src
│ │ │ ├───main
│ │ │ │ ├───java
│ │ │ │ │ │ App.java
│ │ │ │ │ │
│ │ │ │ │ └───com
│ │ │ │ │ └───example
│ │ │ │ │ └───classloading
│ │ │ │ │ JarClassLoader.java
│ │ │ │ │ Module.java
│ │ │ │ │
│ │ │ │ └───resources
│ │ │ └───test
│ │ │ └───java
│ │ └───target
│ │ ├───classes
│ │ │ │ App.class
│ │ │ │
│ │ │ └───com
│ │ │ └───example
│ │ │ └───classloading
│ │ │ JarClassLoader.class
│ │ │ Module.class
│ │ │
│ │ └───generated-sources
│ │ └───annotations
│ └───weather-module
│ │ pom.xml
│ │
│ ├───src
│ │ ├───main
│ │ │ ├───java
│ │ │ │ └───com
│ │ │ │ └───example
│ │ │ │ └───classloading
│ │ │ │ Module.java
│ │ │ │ Weather.java
│ │ │ │
│ │ │ └───resources
│ │ └───test
│ │ └───java
│ └───target
│ │ weather-module-1.0-SNAPSHOT.jar
│ │
│ ├───classes
│ │ │ Module.class
│ │ │ Weather.class
│ │ │
│ │ └───com
│ │ └───example
│ │ └───classloading
│ │ Module.class
│ │ Weather.class
│ │
│ ├───maven-archiver
│ │ pom.properties
│ │
│ └───maven-status
│ └───maven-compiler-plugin
│ ├───compile
│ │ └───default-compile
│ │ createdFiles.lst
│ │ inputFiles.lst
│ │
│ └───testCompile
│ └───default-testCompile
│ inputFiles.lst
│
└───
更新:
我在JarClassLoader cacheClasses()
if (match(normalize(jarEntry.getName()), packageName))
到
if (match(normalize(jarEntry.getName()), packageName)
&& !normalize(jarEntry.getName()).contains("Module"))
这是解决方法.如何以正确的方式做到这一点?
It is workaround. How to do it in a right way?
更新:据我了解，可以从模块Weather中删除Module接口，然后将菜单"模块声明为天气模块的依赖项" @Costi Ciudatu .
Update: as I understand it is possible to delete Module interface from module Weather then "declare the "menu" module as a dependency for the weather module" @Costi Ciudatu.
现在我有以下pom.xml文件:
菜单模块
<?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">
<parent>
<artifactId>classloading</artifactId>
<groupId>java-tasks</groupId>
<version>1.0-SNAPSHOT</version>
</parent>
<modelVersion>4.0.0</modelVersion>
<artifactId>menu</artifactId>
<dependencies>
<dependency>
<groupId>org.apache.logging.log4j</groupId>
<artifactId>log4j-api</artifactId>
<version>2.8.1</version>
</dependency>
<dependency>
<groupId>org.apache.logging.log4j</groupId>
<artifactId>log4j-core</artifactId>
<version>2.8.1</version>
</dependency>
</dependencies>
</project>
天气模块
<?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>java-tasks</groupId>
<artifactId>weather-module</artifactId>
<version>1.0-SNAPSHOT</version>
<dependencies>
<dependency>
<groupId>java-tasks</groupId>
<artifactId>menu</artifactId>
<version>1.0-SNAPSHOT</version>
</dependency>
</dependencies>
</project>
类加载
<?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>java-tasks</groupId>
<artifactId>classloading</artifactId>
<packaging>pom</packaging>
<version>1.0-SNAPSHOT</version>
<modules>
<module>weather-module</module>
<module>menu-module</module>
</modules>
</project>
问题:我尝试打包weather-module并收到错误消息:
Problem: I tried to package weather-module and got error:
[INFO] Scanning for projects...
[INFO]
[INFO] ------------------------------------------------------------------------
[INFO] Building weather-module 1.0-SNAPSHOT
[INFO] ------------------------------------------------------------------------
[WARNING] The POM for java-tasks:menu:jar:1.0-SNAPSHOT is missing, no dependency information available
[INFO] ------------------------------------------------------------------------
[INFO] BUILD FAILURE
[INFO] ------------------------------------------------------------------------
[INFO] Total time: 0.471 s
[INFO] Finished at: 2017-04-07T09:15:38+03:00
[INFO] Final Memory: 8M/245M
[INFO] ------------------------------------------------------------------------
[ERROR] Failed to execute goal on project weather-module: Could not resolve dependencies for project java-tasks:weather-module:jar:1.0-SNAPSHOT: Could not find artifact java-tasks:menu:jar:1.0-SNAPSHOT -> [Help 1]
[ERROR]
[ERROR] To see the full stack trace of the errors, re-run Maven with the -e switch.
[ERROR] Re-run Maven using the -X switch to enable full debug logging.
[ERROR]
[ERROR] For more information about the errors and possible solutions, please read the following articles:
[ERROR] [Help 1] http://cwiki.apache.org/confluence/display/MAVEN/DependencyResolutionException
我应该如何配置Maven pom.xml文件以使其正常工作?
How I should configure maven pom.xml files for correct working?
推荐答案
您的天气模块不应包含Module类的副本.
否则，您将获得该类的两个副本，这是ClassCastException的根本原因.
Your weather module should not contain a copy of the Module class.
Otherwise, you end up with two copies of that class, which is the root cause of the ClassCastException.
使天气模块依赖于菜单模块，或将Module类提取到单独的类中.最重要的是，您应该确保在类路径中最终使用Module的单个版本.
Make the weather module depend on the menu module or extract the Module class in a separate one. Bottomline, you should make sure you end up with a single version of Module in your classpath.
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