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[英]With Maven how do I build two versions of a Java project with just one different .java file?
[英]How to load two versions of a class from two different versions of jars in a java maven project?
我有一个Maven项目,该项目依赖于版本“ 2.0”的jar“样本”。 版本“ 2.0”的“示例” jar包含一个名为“ SampleEnum”的枚举。 “ 2.0”版本中的SampleEnum如下所示
public enum SampleEnum {
"HERBIVORES",
"CARNIVORES",
"OMNIVORES"
}
我想从上述maven java项目中的版本“ 1.0”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"
}
我使用以下代码加载该类:
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());
}
}
}
上面程序的实际输出为:
香草草食肉杂草
当我从“ sample-1.0” jar中加载该类时,我期望以下输出:
香草草食肉
我可以知道为什么从依赖的jar而不是指定的jar中加载类的原因吗?
有什么办法可以从Maven项目的1.0版本的jar中加载枚举类?
它从从属jar加载类的原因是因为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,而该类加载器从从属jar加载类。 公共ClassLoader.loadClass(String)
方法调用受保护的ClassLoader.loadClass(String, boolean)
方法,并且记录了该方法的解释以解释始终总是首先搜索父类加载器。 当然,您可以通过URLClassLoader
并重写loadClass(String, boolean)
方法来覆盖此行为。
org.sample.SampleEnum
从样品-1.0.jar
public enum SampleEnum
{
HERBIVORES,
CARNIVORES
}
来自sample-2.0.jar的org.sample.SampleEnum
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]
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