如何从外部目录中的 jar 创建 bean

Question

I have a basic spring application that simply loads a bean and thats it. I know that within the project I can use the annotation @Component within a class to mark it as a bean and the spring application will find it and act accordingly.

However, I am currently trying to place the component in a jar ie plugin.jar in the external directory /Users/me/plugin.jar. I am at a lost for how to get spring to search within that jar for an annotated class instead of the local package. Is this even possible?

I want to be able to load the bean from the jar without the jar being included in the class-path but rather by searching the directory for the jar and dynamically creating the bean based on whatever Jar is in there.

I have used Java reflection and a URLClassLoader for dynamically loading classes from the jar but cannot figure out how to do the same thing in Spring. Any help or direction on how to get started is greatly appreciated. Thanks!

Additional Info: The spring application is meant to be designed so that while it is running, it scans a designated directory for any jars, if it finds a jar it creates a bean from one of classes inside. I cannot find out how to get the spring application to search scan a specified directory for jars to use as components. Everything I find simply references classes within the project.

Answer 1

You can create a Configuration where you initiate the required bean from external jar.

 @Configuration
    public class AppConfig {

        @Bean
        public MyBean myBean() {
            return new YourExternalClassName();
        }

    }

Answer 2

Use the @ComponentScan annotation on a configuration class, and specify your external component's package:

@Configuration
@ComponentScan(basePackages={"com.foo.bar"})
public class Config { }

You can also define a meta annotion in your external jar:

@Retention(value=RUNTIME)
@Target(value=TYPE)
@ComponentScan(basePackages={"com.foo.bar"})
public @interface EnableExternalFoo { }

And apply this to a configuration class:

@Configuration
@EnableExternalFoo
public class Config { }

Answer 3

If your plugin.jar is a Spring project with @Component, you can just scan the package:

@ComponentScan(basePackages = {"com.exp"}, basePackageClasses = DependencyBasePackageClass.class)

//external.jar
package com.exp;

import org.springframework.stereotype.Component;

@Component
public class ExpBean {
}

If the bean is just a class in the external.jar, you can create Factory or Singleton it and use it as a Bean.

Answer 4

However, I am currently trying to place the component in a jar ie plugin.jar in the external directory /Users/me/plugin.jar. I am at a lost for how to get spring to search within that jar for an annotated class instead of the local package. Is this even possible?

You could add the JAR in the classpath of the application at runtime. But the problem is that at compile time you also need to have the jar as a compilation dependency.
Since you use Spring Boot, it means that you use Maven or Gradle as build tool.
In that case, you don't have to use local directories to specify your external jars otherwise your builds will be reproducible only on your machine and in a very specific state of your machine (the jar has to be in the specified directory). You don't want to defeat the build tool advantages.
A more robust way would be to apply the maven/gradle way:
- define that jar as a maven/gradle artifact
- push it to your repo (local and central if you use that)
- declare the jar in the dependencies of the pom.xml / build.gradle of your application

Then in your application you can declare a bean of the Foo class defining in the JAR such as:

@Configuration
public class ExternalBeanConfig {

        @Bean
        public Foo foo() {
            return new Foo();
        }

}