如何在java maven项目中从两个不同版本的jar中加载一个类的两个版本？

Question

我有一个maven项目，它依赖于版本为"2.0“的jar”样本“。版本为"2.0“的”示例“jar包含一个名为"SampleEnum”的枚举。2.0版本的SampleEnum如下所示

public enum SampleEnum {
    "HERBIVORES",
    "CARNIVORES",
    "OMNIVORES"
}

我想从上面提到的maven java项目的"1.0“版本的"sample”jar中加载"SampleEnum“类。"1.0“版本中的SampleEnum类如下所示。

public enum SampleEnum {
    "HERBIVORES",
    "CARNIVORES"
}

我使用下面的代码来加载这个类：

public class Test
{
    public static void main(String args[]) {
        try {
            URLClassLoader loader1 = new URLClassLoader(new URL[] {new File("sample-1.0.jar").toURL()}, Thread.currentThread().getContextClassLoader());

            Class<?> c1 = loader1.loadClass("SampleEnum");

            for (Object o : c1.getEnumConstants()) {
                System.out.println(o);
            }
        }
        catch(Exception ex)
        {
            System.err.println(ex.getMessage());
        }
    }
}

上述程序的实际输出为：

食草动物、肉食动物、杂食动物

因为我已经从"sample-1.0“jar加载了类，所以我期望得到以下输出：

食草动物肉食动物

我可以知道为什么它从依赖的jar而不是指定的jar加载类吗？

有没有办法在maven项目中从1.0版jar中加载枚举类？

Answer 1

之所以从依赖jar加载类，是因为loader1类加载器将当前线程的上下文类加载器作为其父类加载器，并且该类加载器从依赖jar加载类。公共ClassLoader.loadClass(String)方法调用受保护的ClassLoader.loadClass(String, boolean)方法，文档中记录了该方法，以说明始终首先搜索父类加载器。当然，您可以通过继承URLClassLoader子类并覆盖loadClass(String, boolean)方法来覆盖此行为。

来自org.sample.SampleEnum -1.0.jar的示例

public enum SampleEnum
{
    HERBIVORES,
    CARNIVORES
}

来自org.sample.SampleEnum -2.0.jar的示例

public enum SampleEnum
{
    HERBIVORES,
    CARNIVORES,
    OMNIVORES
}

Main.java

package com.example.app;

import org.sample.SampleEnum;
import java.io.File;
import java.net.URL;
import java.net.URLClassLoader;

import static java.util.Arrays.asList;

public class Main
{
    public static void main(String[] args)
    {
        System.out.println("v2.0: " + asList(SampleEnum.values()));
        try {
            URLClassLoader loader1 = new CustomURLClassLoader(
                    new URL[] { new File("../sample-1.0/target/sample-1.0.jar").toURL() });

            Class<?> c1 = loader1.loadClass("org.sample.SampleEnum");

            System.out.println("v1.0: " + asList(c1.getEnumConstants()));
        } catch (Exception ex) {
            ex.printStackTrace();
        }
    }
}

class CustomURLClassLoader extends URLClassLoader
{
    public CustomURLClassLoader(URL[] urls)
    {
        super(urls);
    }

    protected Class<?> loadClass(String name, boolean resolve)
            throws ClassNotFoundException
    {
        synchronized (getClassLoadingLock(name)) {
            // First, check if the class has already been loaded
            Class<?> c = findLoadedClass(name);
            if (c == null) {
                long t0 = System.nanoTime();

                // First, look in the current ClassLoader
                long t1 = System.nanoTime();
                try {
                    c = findClass(name);
                } catch (ClassNotFoundException ex) {
                    // ClassNotFoundException thrown if class not found
                }

                // this is the defining class loader; record the stats
                sun.misc.PerfCounter.getParentDelegationTime().addTime(t1 - t0);
                sun.misc.PerfCounter.getFindClassTime().addElapsedTimeFrom(t1);
                sun.misc.PerfCounter.getFindClasses().increment();

                // Lastly, look in the parent ClassLoader
                try {
                    if (getParent() != null) {
                        c = super.loadClass(name, resolve);
                    }
                } catch (ClassNotFoundException e) {
                    // ClassNotFoundException thrown if class not found
                    // from the non-null parent class loader
                }

            }
            if (resolve) {
                resolveClass(c);
            }
            return c;
        }
    }
}

输出：

$ java -cp sample-app-2.0.jar:sample-2.0.jar com.example.app.Main
v2.0: [HERBIVORES, CARNIVORES, OMNIVORES]
v1.0: [HERBIVORES, CARNIVORES]

有关更多信息，请参阅https://docs.oracle.com/javase/7/docs/api/java/lang/ClassLoader.html#loadClass(java.lang.String,%20boolean)