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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?
我有一個Maven項目,該項目依賴於版本“ 2.0”的jar“樣本”。 版本“ 2.0”的“示例” jar包含一個名為“ SampleEnum”的枚舉。 “ 2.0”版本中的SampleEnum如下所示
public enum SampleEnum {
"HERBIVORES",
"CARNIVORES",
"OMNIVORES"
}
我想從上述maven java項目中的版本“ 1.0”的“ sample” jar中加載“ SampleEnum”類。 “ 1.0”版本中的SampleEnum類如下所示。
public enum SampleEnum {
"HERBIVORES",
"CARNIVORES"
}
我使用以下代碼加載該類:
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());
}
}
}
上面程序的實際輸出為:
香草草食肉雜草
當我從“ sample-1.0” jar中加載該類時,我期望以下輸出:
香草草食肉
我可以知道為什么從依賴的jar而不是指定的jar中加載類的原因嗎?
有什么辦法可以從Maven項目的1.0版本的jar中加載枚舉類?
它從從屬jar加載類的原因是因為loader1
類加載器將當前線程的上下文類加載器作為其父類加載器,而該類加載器從從屬jar加載類。 公共ClassLoader.loadClass(String)
方法調用受保護的ClassLoader.loadClass(String, boolean)
方法,並且記錄了該方法的解釋以解釋始終總是首先搜索父類加載器。 當然,您可以通過URLClassLoader
並重寫loadClass(String, boolean)
方法來覆蓋此行為。
org.sample.SampleEnum
從樣品-1.0.jar
public enum SampleEnum
{
HERBIVORES,
CARNIVORES
}
來自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;
}
}
}
輸出:
$ 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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