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How to add external JARs to Spring application without restarting JVM?

I have a Spring Boot application which copies external JAR files to a folder, depending on certain conditions. These JARs can contain many Spring components (i.e. classes annotated or meta-annotated with #Component) and the Spring application should be able scan and instantiate for these beans. Is it possible, based on certain conditions, to dynamically load the contents of the JAR files and make them available to the Spring application context? I am fully aware of the security implications this has.
I have read about the different types of Launchers which Spring provides for its executable JAR format, such as JarLauncher and PropertiesLauncher, but it looks like that these launchers do not detect changes to the classpath, but instead only scan the directories once for JAR files.
The following simple application demonstrates the problem:
// .../Application.java
#SpringBootApplication
public class Application {
public static void main(String[] args) {
System.out.println("Please copy JAR files and press Enter ...");
System.in.read();
SpringApplication.run(Application.class, args);
}
}
Replace the default JarLauncher with PropertiesLauncher:
// build.gradle
tasks.named('bootJar') {
manifest {
attributes 'Main-Class': 'org.springframework.boot.loader.PropertiesLauncher',
'Start-Class': 'com.example.customlauncher.Application'
}
}
Specify the location to the external JARs in the properties file of the PropertiesLauncher:
# .../resources/loader.properties
loader.path=file:/path/to/dir
The application is a Spring Initializer Gradle application and packaged by running the bootJar task: ./gradlew bootJar.
It is then started with the following command:
java -jar build/libs/customlauncher-0.0.1-SNAPSHOT.jar
This works if the JAR file is already present at the specified location (/path/to/dir), but it does not work if the java command is executed while the directory is empty and the JAR file is then copied while the app waits for the user to copy the files and press Enter ↲.
There are a couple of related questions, but it looks like they all assume that the JAR files already exist at the time of starting the JVM:
How to put a directory first on the classpath with Spring Boot?
Spring Boot Executable Jar with Classpath
SpringBoot external jar not load
Is there a way to achieve this without too many awkard hacks? Or is recommended to utilize something like OSGi? Am I looking at this completely wrong and there is a better way to have JARs on the classpath that do not need always need loading (if the JAR is "disabled", it should not be loaded/compiled by the JVM, should not be picked up by Spring, etc.)?

It looks like this is possible if the JAR files are copied before starting the Spring application. It feels hackish, but it works. Use at your own risk!
You need two classes, one for bootstrapping the external JARs, which will then start the second via a manually created PropertiesLauncher. The bootstrapping class can be a plain old regular Java class (but it can be a Spring Boot Application too) and only the second class needs to be a SpringBootApplication.
// BootstrapApplication.java
public class BootstrapApplication {
public static void main(String[] args) {
System.out.println("Please copy JAR files and press Enter ...");
System.in.read();
PropertiesLauncher.main(args);
}
}
// Application.java
#SpringBootApplication
public class Application {
public static void main(String[] args) {
SpringApplication.run(Application.class, args);
}
}
In the gradle file, we can switch back to the default JarLauncher, by removing the bootJar task manifest configuration and applying settings via the springBoot configuration block. mainClass will end up as Start-Class in the MANIFEST.MF file.
// build.gradle
springBoot {
mainClass = 'com.example.customlauncher.BootstrapApplication'
}
In the properties file for the loader, a new property needs to be set, which points to the real application class. The settings in this file are only picked up by PropertiesLauncher and ignored by JarLauncher. In other words: JarLauncher delegates to Start-Class from the manifest file and PropertiesLauncher delegates to loader.main from its properties file.
# .../resources/loader.properties
loader.path=file:/path/to/dir
loader.main=com.example.customlauncher.Application
Spring (Boot) will first call the main method of BootstrapApplication, as specified in the MANIFEST.MF file (controlled via springBoot configuration block in the build.gradle file). In the implementation of this main method, a new PropertiesLauncher is created with the main class set to the "real" application (i.e. Application).
Executing the application is still done via the same invocation:
java -jar build/libs/customlauncher-0.0.1-SNAPSHOT.jar
Any JAR files added to /path/to/dir after the JVM has started, but before calling PropertiesLauncher#main in BootstrapApplication are then available in the classpath and application context as seen from Application.

How to convert a .war file into a JAR file?

I did a project based on J2EE framework. Using spring/ hibernate technologies. And used Tomcat as my servlet container. I want to create a JAR file and make my web project executable. This question is asked before but answers are ambiguous.

Have a look at Spring Boot, this will help you create a jar that can be started with
java -jar <your-jar-file>
It would not need to get deployed into a servlet container, instead it will run with an embedded tomcat. You'd need to install a java runtime wherever you want to run it, though.
See this link for more information.

To convert war file to jar in the spring boot, you need to do 3 steps.
convert war to jar in the pom.xml
<packaging>jar</packaging>
extends SpringBootServletInitializer
override configure
public class Your_Application_Name extends SpringBootServletInitializer{
#Override
protected SpringApplicationBuilder configure(SpringApplicationBuilder springApplicationBuilder){
return springApplicationBuilder.sources(Your_Application_Name.class);
}
public static void main(String[] args) {
SpringApplication.run(Your_Application_Name.class, args);
}

Converting a Spring Boot JAR Application to a WAR

I have tried to run "Converting a Spring Boot JAR Application to a WAR" application by converting into a war package as instructed...and since it was mentioned that void main() is no longer needed,I removed it and tried to build it using gradle but it throws error unable to find main class.
The content of class after my modification is as below
Application.java
#Configuration
#EnableAutoConfiguration
#ComponentScan
public class Application {
}
what is the mistake am I making?

If you don't want an executable war then remove the Spring Boot plugin. If you do, leave it in, and keep the main().

netbeans 7-8, inject of remote ejb

Hi, I have an EJB project called "service-ejb" with this:
#Stateless
#Remote(ServiceRemote.class)
public class Services implements ServiceLocal, ServiceRemote {
[...business code...]
}
the I have the local interface, in the same project:
#Local
public interface ServiceLocal { }
and the remote interface, in a class library project called "service-lib":
#Remote
public interface ServiceRemote {
public boolean checkIfOk();
}
I can deploy it without problem, alone or in a java EE application. The point is that I don't understand how to tell NetBeans that I wish to call that beans from another application. For example I have another java EE project with a war component, where inside a servlet I wrote:
#EJB
private ServiceRemote serviceTest;
but of course it will fail compiling, so I tried with:
InitialContext ic = new InitialContext();
ServiceRemote serviceTest = (ServiceRemote) ic.lookup("ServiceRemote");
with no luck... Where in NetBeans I can tell it to use the "service-lib" as a reference? I don't want it to be added as library and then deployed with the ear, I only want NetBeans to compile correctly the code.
Sorry if the question sound silly, but I've read the documentation and I don't understant what I'm missing...
--- edit ---
I'll try to be more clear. "service-ejb" reference "service-lib", so I've deployed "service-ejb" to glassfish. Correctly, I have:
glassfish_applications_directory $ find -name "service*"
./__internal/service-ejb
./__internal/service-ejb/service-ejb.jar
./service-ejb
./service-ejb/com/tecytal/components/email/beans/Service.class
./service-ejb/com/tecytal/components/email/interfaces/local/ServiceLocal.class
./service-ejb/service-lib.jar
Then I open a java EE project, let's call it "myEngine" with a war module "myEngine-war". I've tried to add to "myEngine-war" a reference to "service-lib", of course, and in this case it compile well. The point is that I can do everything, tell to netbeans NOT to package the "service-lib" with the war, but when I deploy "myEngine" I get:
glassfish_applications_directory $ find -name "service*"
./myEngine/lib/service-lib.jar
./__internal/service-ejb
./__internal/service-ejb/service-ejb.jar
./service-ejb
./service-ejb/com/tecytal/components/email/beans/Service.class
./service-ejb/com/tecytal/components/email/interfaces/local/ServiceLocal.class
./service-ejb/service-lib.jar
I DON'T want to have TWO service-lib.jar in my server, one in the service-ejb and one in myEngine, I don't understand how I can use a remote ebj in netbeans telling to him NOT TO deploy the same lib 3214899213 times :)

I recommend to add the remote service interface and all classes it references (including exceptions) in a separate jar. Then in the other 2 projects declare a dependcy to this jar.
I don't want it to be added as library and then deployed with the ear
I recommed to add only the service-lib contain the interface etc as a library and deploy it with the ear, not the whole service-ejb containing the implementation.

Spring Dependency Injection and Plugin Jar

I have web application running with a default impl of a backend service. One should be able to implement the interface and drop the jar into the plugins folder (which is not in the apps classpath). Once the server is restarted, the idea is to load the new jar into the classloader, and have it take part in dependency injection. I am using Spring DI using #Autowired. The new plugin service impl will have #Primary annotation. So given two impls of the interface, the primary should be loaded.
I got the jar loaded into the classloader and can invoke the impl manually. But I haven't been able to get to to participate in the Dependency Injection, and have it replace the default impl.
Here's a simplified example:
#Controller
public class MyController {
#Autowired
Service service;
}
//default.jar
#Service
DefaultService implements Service {
public void print() {
System.out.println("printing DefaultService.print()");
}
}
//plugin.jar not in classpath yet
#Service
#Primary
MyNewService implements Service {
public void print() {
System.out.println("printing MyNewService.print()");
}
}
//For lack of better place, I loaded the plugin jar from the ContextListener
public class PluginContextLoaderListener extends org.springframework.web.context.ContextLoaderListener {
#Override
protected void customizeContext(ServletContext servletContext,
ConfigurableWebApplicationContext wac) {
System.out.println("Init Plugin");
PluginManager pluginManager = PluginManagerFactory.createPluginManager("plugins");
pluginManager.init();
//Prints the MyNewService.print() method
Service service = (Service) pluginManager.getService("service");
service.print();
}
}
<listener>
<listener-class>com.plugin.PluginContextLoaderListener</listener-class>
</listener>
Even after I have loaded the jar into the classloader, DefaultService is still being injected as service. Any idea how I get the plugin jar to participate into the spring's DI lifecycle?
Edited:
To put it simply, I have a war file that has a few plugin jars in a plugins directory inside the war. Based on a value from a configuration file that the app looks at, when the app is started, I want to load that particular plugin jar and run the application with it. That way, I can distribute the war to anyone, and they can choose which plugin to run based on a config value without having to to repackage everything. This is the problem I am trying to solve.

It seems like all You need is to create the Spring ApplicationContext properly. I think it's possible without classpath mingling. What matters most are the locations of the Spring configuration files within the classpath. So put all Your plugin jar's into WEB-INF/lib and read on.
Let's start with the core module. We'll make it to create it's ApplicationContext from files located at classpath*:META-INF/spring/*-corecontext.xml.
Now we'll make all plugins to have their config files elsewhere. I.e. 'myplugin1' will have its config location like this: classpath*:META-INF/spring/*-myplugin1context.xml. And anotherplugin will have the configs at classpath*:META-INF/spring/*-anotherplugincontext.xml.
What You see is a convension. You can also use subdirectiries if You like:
core: classpath*:META-INF/spring/core/*.xml
myplugin1: classpath*:META-INF/spring/myplugin1/*.xml
anotherplugin: classpath*:META-INF/spring/anotherplugin/*.xml
What matters is that the locations have to be disjoint.
All that remains is to pass the right locations to the ApplicationContext creator. For web applications the right place for this would be to extend the ContextLoaderListener and override the method customizeContext(ServletContext, ConfigurableWebApplicationContext).
All that remains is to read Your config file (its location can be passed as servlet init parameter). Than You need to construct the list of config locations:
String locationPrefix = "classpath*:META-INF/spring/";
String locationSiffix = "/*.xml";
List<String> configLocations = new ArrayList<String>();
configLocations.add(locationPrefix + "core" + locationSiffix);
List<String> pluginsTurnedOn = getPluginsTurnedOnFromConfiguration();
for (String pluginName : pluginsTurnedOn) {
configLocations.add(locationPrefix + pluginName + locationSiffix);
}
applicationContext.setConfigLocations(configLocations.toArray(new String[configLocations.size()]));
This way You can easily manage what is and what is not loaded into Spring ApplicationContext.
Update:
To make it work there's one more hidden assumption I made that I'm about to explain now. The base package of the core module and each plugin should also be disjoint. That is i.e.:
com.mycompany.myapp.core
com.mycompany.myapp.myplugin1
com.mycompany.myapp.anotherplugin
This way each module can use <context:componet-scan /> (on equivalent in JavaConfig) easily to add classpath scanning for it's own classes only. The core module should not contain any package scanning of any plugin packages. The plugins should extend configuration of ApplicationContext to add their own packages to classpath scanning.

If you restart the server, I see no reason why you can't just add the JAR to the WEB-INF/lib and have it in the CLASSPATH. All the complication of a custom class loader and context listener goes away, because you treat it just like any other class under Spring's control.
If you do it this way because you don't want to open or modify a WAR, why not put it in the server /lib directory? Let the server class loader pick it up. This makes all plugin classes available to all deployed apps.
The answer depends on how important the separate /plugin directory is. If it's key to the solution, and you can't add the JAR to the server's /lib directory, then that's that. I've got nothing. But I think it'd be worthwhile to at least revisit the solution you have to make sure that it's the only way to accomplish what you want.