I am trying to use a custom class loader to load all the dependencies needed for the application. I've implemented the customerClassLoader following the site: https://www.javacodegeeks.com/2013/03/java-handmade-classloader-isolation.html
However, I dont understand how to tell my application to use the custom classLoader whenever needed.
For instance: Lets say, I have a method to make http request like below. How can I tell the application to use the custom classLoader to load the required jars?
private HttpResponse get() {
HttpClient client = HttpClientBuilder.create().build();
HttpGet request = new HttpGet(url);
HttpResponse response = client.execute(request);
return response;
}
Java uses ClassLoader
implicitly when you use new
, import
keyword, the jvm will use the current class's classloader to load the dependent classes, so you can use the custom classloader to load a bootstrap
class explicitly by using classloader.loadclass
, and the bootstrap
just runs a method belonging to your target class instance. An example follows.
There is a class Target
that depends on the class DateFormatter
which is included in the spring-context
, and has a method named start
.
import org.springframework.format.datetime.DateFormatter;
public class Target {
private static DateFormatter dateFormatter;
public void start(){
System.out.println(this.getClass().getClassLoader());
dateFormatter=new DateFormatter();
System.out.println(dateFormatter);
}
}
Next, we compile and package the above code as a jar named target.jar
, which is stored at D:\\test\\target.jar
.
Next, we declare a class BootStrap
in another jar that will call the method start
of Target
instance. The BootStrap
class will dynamically load the target.jar
and spring-context
jar files by the same classloader
which is a URLClassLoader
instance. Because of this, the method start
in Target
instance can access the DateFormatter
class that is defined in spring-context
.
public class BootStrap {
public static void main(String[] args) throws Exception{
URL url = new URL("http://maven.aliyun.com/nexus/content/groups/public/org/springframework/spring-context/4.3.1.RELEASE/spring-context-4.3.1.RELEASE.jar?spm=0.0.0.0.kG1Pdw&file=spring-context-4.3.1.RELEASE.jar");
URL url2= (new File("D:\\test\\target.jar").toURI().toURL());
URLClassLoader classLoader = new URLClassLoader(new URL[]{url,url2});
Class<?> clz = classLoader.loadClass("com.zhuyiren.Target");
Object main = clz.newInstance();
Method test = clz.getMethod("start");
test.invoke(main);
}
}
Finally, run the BootStrap
main method. There are two important thing:
BootStrap
class and Target
class don't belong to a same jar file.target.jar
is not stored in CLASSPATH
path.These 2 point can make sure that the AppClassLoader
can not find and load the Target
class. Because of the mechanism of class loader, jvm will use the custom load the Target
. Of course, you can guarantee it by changing the URLClassLoader classLoader = new URLClassLoader(new URL[]{url,url2});
to URLClassLoader classLoader = new URLClassLoader(new URL[]{url, url2}, ClassLoader.getSystemClassLoader().getParent());
And we can see the result:
java.net.URLClassLoader@e9e54c2
org.springframework.format.datetime.DateFormatter@4dd8dc3
That means we can access the DateFormatter
instance which is defined in spring-context
jar file successfully, while the spring-context
is not stored in CLASSPATH
, but we are using the custom clasloader to load and use it.