Creating Spring context in a library

Question

I am developing a library which will be used from a non-Spring application. However, I need to use another library which is developed for a work in Spring context and want to also use Spring features like dependency injection. So, I need to create a Spring ApplicationContext in my library and include dependency library into it. To demonstrate the situation, I have an example project with two modules:

Root project pom.xml (lib-with-spring-dep):

<?xml version="1.0" encoding="UTF-8"?>
<project xmlns="http://maven.apache.org/POM/4.0.0"
         xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance"
         xsi:schemaLocation="http://maven.apache.org/POM/4.0.0 http://maven.apache.org/xsd/maven-4.0.0.xsd">
    <modelVersion>4.0.0</modelVersion>

    <groupId>org.example</groupId>
    <artifactId>lib-with-spring-dep</artifactId>
    <version>1.0-SNAPSHOT</version>
    <packaging>pom</packaging>

    <properties>
        <maven.compiler.source>8</maven.compiler.source>
        <maven.compiler.target>8</maven.compiler.target>
    </properties>

    <modules>
        <module>lib</module>
        <module>spring-lib</module>
    </modules>

    <dependencies>
        <dependency>
            <groupId>junit</groupId>
            <artifactId>junit</artifactId>
            <version>4.13.2</version>
            <scope>test</scope>
        </dependency>
        <dependency>
            <groupId>org.example</groupId>
            <artifactId>lib</artifactId>
            <version>1.0-SNAPSHOT</version>
            <scope>test</scope>
        </dependency>
    </dependencies>
</project>

pom.xml (lib):

<?xml version="1.0" encoding="UTF-8"?>
<project xmlns="http://maven.apache.org/POM/4.0.0"
         xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance"
         xsi:schemaLocation="http://maven.apache.org/POM/4.0.0 http://maven.apache.org/xsd/maven-4.0.0.xsd">
    <modelVersion>4.0.0</modelVersion>

    <parent>
        <artifactId>lib-with-spring-dep</artifactId>
        <groupId>org.example</groupId>
        <version>1.0-SNAPSHOT</version>
    </parent>

    <artifactId>lib</artifactId>

    <properties>
        <maven.compiler.source>8</maven.compiler.source>
        <maven.compiler.target>8</maven.compiler.target>
    </properties>

    <dependencies>
        <dependency>
            <groupId>org.example</groupId>
            <artifactId>spring-lib</artifactId>
            <version>1.0-SNAPSHOT</version>
        </dependency>
        <dependency>
            <groupId>org.springframework</groupId>
            <artifactId>spring-beans</artifactId>
            <version>5.3.13</version>
        </dependency>
    </dependencies>
</project>

pom.xml (spring-lib):

<?xml version="1.0" encoding="UTF-8"?>
<project xmlns="http://maven.apache.org/POM/4.0.0"
         xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance"
         xsi:schemaLocation="http://maven.apache.org/POM/4.0.0 http://maven.apache.org/xsd/maven-4.0.0.xsd">
    <modelVersion>4.0.0</modelVersion>

    <parent>
        <artifactId>lib-with-spring-dep</artifactId>
        <groupId>org.example</groupId>
        <version>1.0-SNAPSHOT</version>
    </parent>

    <artifactId>spring-lib</artifactId>

    <properties>
        <maven.compiler.source>8</maven.compiler.source>
        <maven.compiler.target>8</maven.compiler.target>
    </properties>

    <dependencies>
        <dependency>
            <groupId>org.springframework</groupId>
            <artifactId>spring-context</artifactId>
            <version>5.3.13</version>
        </dependency>
    </dependencies>
</project>

SpringService.java:

package org.example.springlib;

import org.springframework.stereotype.Service;

@Service
public class SpringService {

    public String getMessage() {
        return "Message";
    }
}

Executor.java:

package org.example.lib;

import org.example.springlib.SpringService;
import org.springframework.beans.factory.annotation.Autowired;
import org.springframework.stereotype.Component;

@Component
public class Executor {

    @Autowired
    private SpringService service;

    public String getMessageFromService() {
        return service.getMessage();
    }
}

TestExecutor.java:

import static org.hamcrest.CoreMatchers.is;

import org.example.lib.Executor;
import org.junit.Assert;
import org.junit.Test;

public class TestExecutor {

    @Test
    public void testSpringContext() {
        String message = new Executor().getMessageFromService();
        Assert.assertThat(message, is("Message"));
    }
}

Running the test gives the exception:

java.lang.NullPointerException
    at org.example.lib.Executor.getMessageFromService(Executor.java:14)
    at TestExecutor.testSpringContext(TestExecutor.java:11)
    at sun.reflect.NativeMethodAccessorImpl.invoke0(Native Method)
    at sun.reflect.NativeMethodAccessorImpl.invoke(NativeMethodAccessorImpl.java:62)
    at sun.reflect.DelegatingMethodAccessorImpl.invoke(DelegatingMethodAccessorImpl.java:43)
    at java.lang.reflect.Method.invoke(Method.java:498)
    at org.junit.runners.model.FrameworkMethod$1.runReflectiveCall(FrameworkMethod.java:59)
    at org.junit.internal.runners.model.ReflectiveCallable.run(ReflectiveCallable.java:12)
    at org.junit.runners.model.FrameworkMethod.invokeExplosively(FrameworkMethod.java:56)
    at org.junit.internal.runners.statements.InvokeMethod.evaluate(InvokeMethod.java:17)
    at org.junit.runners.ParentRunner$3.evaluate(ParentRunner.java:306)
    at org.junit.runners.BlockJUnit4ClassRunner$1.evaluate(BlockJUnit4ClassRunner.java:100)
    at org.junit.runners.ParentRunner.runLeaf(ParentRunner.java:366)
    at org.junit.runners.BlockJUnit4ClassRunner.runChild(BlockJUnit4ClassRunner.java:103)
    at org.junit.runners.BlockJUnit4ClassRunner.runChild(BlockJUnit4ClassRunner.java:63)
    at org.junit.runners.ParentRunner$4.run(ParentRunner.java:331)
    at org.junit.runners.ParentRunner$1.schedule(ParentRunner.java:79)
    at org.junit.runners.ParentRunner.runChildren(ParentRunner.java:329)
    at org.junit.runners.ParentRunner.access$100(ParentRunner.java:66)
    at org.junit.runners.ParentRunner$2.evaluate(ParentRunner.java:293)
    at org.junit.runners.ParentRunner$3.evaluate(ParentRunner.java:306)
    at org.junit.runners.ParentRunner.run(ParentRunner.java:413)
    at org.junit.runner.JUnitCore.run(JUnitCore.java:137)
    at com.intellij.junit4.JUnit4IdeaTestRunner.startRunnerWithArgs(JUnit4IdeaTestRunner.java:69)
    at com.intellij.rt.junit.IdeaTestRunner$Repeater.startRunnerWithArgs(IdeaTestRunner.java:33)
    at com.intellij.rt.junit.JUnitStarter.prepareStreamsAndStart(JUnitStarter.java:235)
    at com.intellij.rt.junit.JUnitStarter.main(JUnitStarter.java:54)

How can I create a Spring context in the lib library? There was also an answered question How does one instantiate Spring in the context of a library? , but it was almost 12 years ago, and I hope there are some new options to achieve this.

Answer 1

I have found an answer here Designing a Java library with Spring . I have added new class BeanFactory to the lib module:

package org.example.lib;

import org.springframework.context.ApplicationContext;
import org.springframework.context.annotation.AnnotationConfigApplicationContext;
import org.springframework.context.annotation.ComponentScan;
import org.springframework.context.annotation.Configuration;

@Configuration
@ComponentScan(basePackages = "org.example")
public class BeanFactory {

    private static ApplicationContext context;
    
    private static ApplicationContext getContext() {
         if (context == null) {
             context = new AnnotationConfigApplicationContext(BeanFactory.class);
         }
         return context;
    }
    
    public static <T> T getBean(Class<T> requiredType) {
        return getContext().getBean(requiredType);
    }
}

and modified the Executor class:

package org.example.lib;

import org.example.springlib.SpringService;

public class Executor {

    public String getMessageFromService() {
        SpringService service = BeanFactory.getBean(SpringService.class);
        return service.getMessage();
    }
}

Now the BeanFactory serves simultaneously as an entry point for accessing the ApplicationContext and as a configuration class. The configuration can be a separate class as well.