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spring dependency injection is not working in a class annotated with #WebListener

I tried constructor based Dependency injection in TestAppListener class which implements ServletContextListener.
I got this error
Exception sending context initialized event to listener instance of class [com.example.listener.TestAppListener].
I searched stack overflow but I couldn't find any solution for this scenario. Please any one help me in sort out this. I placed Implementation classes in META-INF.services folder also. My understanding is there is some problem in constructor dependency injection but this way of DI is need for my situation, because in real time I want to create datasource connection inside startup method.
Find all my classes below which i'm using,
#WebListener
public class TestAppListener implements ServletContextListener {
private static TestDao dao;
public TestAppListener(TestDao dao){
this.dao = dao;
}
public TestAppListener(){}
#Override
public void contextInitialized(ServletContextEvent sce) {
dao = ServiceLoader.load(TestDao.class).iterator().next();
dao.startUp();
System.out.println("Context initialized method called");
}
#Override
public void contextDestroyed(ServletContextEvent sce) {
System.out.println("Context destroyed method called");
dao.shutDown();
}
}
public interface TestDao {
void startUp();
void shutDown();
}
public class TestDaoImpl implements TestDao {
#Override
public void startUp() {
System.out.println("Database is initialized");
}
#Override
public void shutDown() {
System.out.println("Database is initialized");
}
}
#Configuration
public class SpringConfig {
public SpringConfig() {
}
#Bean
public ServletListenerRegistrationBean<ServletContextListener> listenerRegistrationBean() {
ServletListenerRegistrationBean<ServletContextListener> bean = new ServletListenerRegistrationBean<>();
bean.setListener(new TestAppListener());
return bean;
}
}

The Servlet #WebListeners are handled by Servlet containers(tomcat/Jetty) when the container is starting. So they know nothing about Spring.
For more details, see discussion in this issue.
A workaround solution is replace the #WebListener with Spring's #Component, thus you can inject Spring beans(declare your Dao as spring beans) into the listeners directly.
BTW, you have to add #ServletComponentScan on your Spring Boot application class to activate it.
I created an example project to demo #WebServlet, #WebFilter and #WebListener.

Change private static TestDao dao to private final TestDao dao. Spring doesn't allow statics to be used as targets for injection. Also, your TestDaoImpl class needs to be a component or a bean defined in a Spring configuration file.

Creating an object of an #Service bean, is it possible?

i have an restful application with all the #Service,#RestController,#Repository beans. and im autowiring required beans.
Now i want to use the #service class in another class that is not managed by spring, is that possible?
These 2 classes are also in 2 diffrent maven projects if that makes any diffrence
i have tried creating a new object , as expected to no awail.
i have also tried creating diffrent constructors also to no awail.
i have googled for some anwsers but havent found any so now i turn to you experts ;)
The class i want to use!
#Service
public class ProductService {
ProductRepository repository;
#Autowired
public ProductService(ProductRepository repository){
this.repository = repository;
}
}
the Restcontroller
#RestController
#RequestMapping(path = "/product")
public class ProductResource {
#Autowired
ProductService service;
}
Repo
public interface ProductRepository extends JpaRepository<Product,Long> {
}
Here is where i want to create the service.
public static void main(String[] args) {
String chromeDriver = args[0];
String method = args[1];
String domainName = args[2];
ProductService service = new ProductService();
System.setProperty("webdriver.chrome.driver", chromeDriver);
Runner runner = new Runner(method,domainName);
runner.run();
}

As far as I know, you can instantiate a Spring-managed class in a non-Spring-managed class as long as the Spring-managed class (in this case the service) doesn't contain any Spring dependencies in it, as those will remain null (non initialized) because they are never gonna be injected by Spring.
Your service looks good to me because the ProductRepository field is not #autowired, but in your code you're creating the ProductService like this:
ProductService service = new ProductService();
While this parameterless constructor doesn't exist in the ProductService class:
#Service
public class ProductService
{
ProductRepository repository;
#Autowired
public ProductService(ProductRepository repository)
{
this.repository = repository;
}
}

I think this is not possible (I mean, it technically is, but it is not going to work).
To have access to your service class your second application needs to have access to the same Spring Context.
This is because Spring Context in your base application defines necessary dependencies that your second application doesn't have access to. One of the examples for this is ProductRepository repository (which is a way Spring Boot app will talk with the database), have no Spring Context and thus information on DB location and connection URL.
In your case you need to replicate the class and mechanism to connect to DB with DB configuration in your second project.

There's a workaround to "inject" dependencies manually. Create a managed component that stores the reference to the ApplicationContext in a static variable, so that you can access from non-managed classes:
#Component
public class ApplicationContextProvider implements ApplicationContextAware {
private static ApplicationContext applicationContext;
public static ApplicationContext applicationContext() {
return applicationContext;
}
#Override
public void setApplicationContext(ApplicationContext applicationContext) {
ApplicationContextProvider.applicationContext = applicationContext;
}
}
public class NoManagedClass {
private final ManagedClass bean;
public NoManagedClass() {
this.bean = ApplicationContextProvider.applicationContext().getBean(ManagedClass.class);
}
}
However, be aware that your non-managed objects should be created after the application context is set. You still have to start your Spring application in the main method so that the beans are initialized.

Not able to get application context object using ApplicationContextAware

i have a microservice application developed using spring boot. i also have a project under under the same application which contains some java classes (non spring classes). i am trying to use some of the beans which are available in the spring container in this non java class using the ApplicationContextAware approach. when i debug the code during the bootrun, i can see the setApplicationContext(ApplicationContext ctx) is getting call and the context is getting set.
when from inside my non spring java class when i tried to get the instance of the context using the public static getApplicationContext(), i am getting null for context.
below is the sample example i had used.
#Component
public class ApplicationContextProvider implements ApplicationContextAware {
private static ApplicationContext context;
public ApplicationContext getApplicationContext() {
return context;
}
#Override
public void setApplicationContext(ApplicationContext ctx) {
context = ctx;
}
}
this is how i try to get the instance
ApplicationContext c = ApplicationContextProvider.getApplicationContext();
i am not able to figure it out what is missing here, as i am using spring boot, i dont think there is a need to configure any bean in xml.

This worked for me with your code. I could get the application start time after fetching the provider from the context.
#RestController
public class SampleController {
#Inject
ApplicationContextProvider provider;
#RequestMapping(value = "..", method = RequestMethod.GET)
public ResponseEntity<String> someMethod() throws IOException {
ApplicationContext c = provider.getApplicationContext();
System.out.println(c.getApplicationName());
c.getStartupDate()
}
}

Pretty way to inject Spring properties to object not managed by Spring

I have a Spring application and I'm using a third party library. There's no constructor, everything is configured and instantiated inside of this library.
I can add some custom behavior by creating a class. What I need is to add my Spring properties to this class.
Here's the class:
public static class CustomClass implements ExternalClass {
#Override
public Object create() {
//Here I would like to inject my properties.
}
}
I'm looking for a pretty approach.

Use the #Configurable annotation on this class to make it possible to inject Environment or directly a property via #Value as usual. This is the documentation of this annotation.
#Configurable
public static class CustomClass implements ExternalClass {
//..
//inject Environment to get property or via #Value as usual
//..
}

create this class in your project
#Component public class IOC implements ApplicationContextAware {
private static ApplicationContext ctx;
#Override
public void setApplicationContext(ApplicationContext applicationContext) throws BeansException {
this.ctx = applicationContext;
}
public static ApplicationContext getCtx() {
return ctx;
}
}
then call
Environment environment = IOC.getCtx().getEnvironment();
in the create method to get your properties

How to get bean using application context in spring boot

I am developing a SpringBoot project and I want to get the bean by its name using applicationContext. I have tried many solution from web but could not succeed. My Requirement is that I have a controller
ControllerA
and inside the controller I have a method getBean(String className). I want to get instance of registered bean. I have hibernate entities and I want to get an instance of the bean by passing the name of class only in getBean method.
Please help if someone know the solution.

You can Autowire the ApplicationContext, either as a field
#Autowired
private ApplicationContext context;
or a method
#Autowired
public void context(ApplicationContext context) { this.context = context; }
Finally use
context.getBean(SomeClass.class)

You can use ApplicationContextAware.
ApplicationContextAware:
Interface to be implemented by any object that wishes to be notified
of the ApplicationContext that it runs in. Implementing this interface
makes sense for example when an object requires access to a set of
collaborating beans.
There are a few methods for obtaining a reference to the application context. You can implement ApplicationContextAware as in the following example:
package hello;
import org.springframework.beans.BeansException;
import org.springframework.context.ApplicationContext;
import org.springframework.context.ApplicationContextAware;
#Component
public class ApplicationContextProvider implements ApplicationContextAware {
private ApplicationContext applicationContext;
#Override
public void setApplicationContext(ApplicationContext applicationContext) throws BeansException {
this.applicationContext = applicationContext;
}
public ApplicationContext getContext() {
return applicationContext;
}
}
Update:
When Spring instantiates beans, it looks for ApplicationContextAware implementations, If they are found, the setApplicationContext() methods will be invoked.
In this way, Spring is setting current applicationcontext.
Code snippet from Spring's source code:
private void invokeAwareInterfaces(Object bean) {
.....
.....
if (bean instanceof ApplicationContextAware) {
((ApplicationContextAware)bean).setApplicationContext(this.applicationContext);
}
}
Once you get the reference to Application context, you get fetch the bean whichever you want by using getBean().

actually you want to get the object from the Spring engine, where the engine already maintaining the object of your required class at that starting of the spring application(Initialization of the Spring engine).Now the thing is you just have to get that object to a reference.
in a service class
#Autowired
private ApplicationContext context;
SomeClass sc = (SomeClass)context.getBean(SomeClass.class);
now in the reference of the sc you are having the object.
Hope explained well. If any doubt please let me know.

Even after adding #Autowire if your class is not a RestController or Configuration Class, the applicationContext object was coming as null. Tried Creating new class with below and it is working fine:
#Component
public class SpringContext implements ApplicationContextAware{
private static ApplicationContext applicationContext;
#Override
public void setApplicationContext(ApplicationContext applicationContext) throws
BeansException {
this.applicationContext=applicationContext;
}
}
you can then implement a getter method in the same class as per your need to get the bean. Like:
applicationContext.getBean(String serviceName,Interface.Class)

Using SpringApplication.run(Class<?> primarySource, String... arg) worked for me. E.g.:
#SpringBootApplication
public class YourApplication {
public static void main(String[] args) {
ConfigurableApplicationContext context = SpringApplication.run(YourApplication.class, args);
}
}

As an alternative approach you can use ConfigurableApplicationContext to get bean of any class which is annotated with #Component, #Repository or #Service.
Let's say you want to get a bean of the class BaseComponent :
#Service
public class BaseComponent {
public String getMessage() {
return "hello world";
}
}
Now you can use ConfigurableApplicationContext to get the bean:
#Component
public class DemoComponent {
#Autowired
ConfigurableApplicationContext applicationContext;
public BaseComponent getBeanOfBaseComponent() {
return applicationContext.getBean(BaseComponent.class);
}
}

You can use the ApplicationContextAware class that can provide the application context.
public class ApplicationContextProvider implements ApplicationContextAware {
private static ApplicationContext ctx = null;
public static ApplicationContext getApplicationContext() {
return ctx;
}
#Override
public void setApplicationContext(final ApplicationContext ctx) throws BeansException {
ApplicationContextProvider.ctx = ctx;
}
/**
* Tries to autowire the specified instance of the class if one of the specified
* beans which need to be autowired are null.
*
* #param classToAutowire the instance of the class which holds #Autowire
* annotations
* #param beansToAutowireInClass the beans which have the #Autowire annotation
* in the specified {#classToAutowire}
*/
public static void autowire(Object classToAutowire, Object... beansToAutowireInClass) {
for (Object bean : beansToAutowireInClass) {
if (bean == null) {
ctx.getAutowireCapableBeanFactory().autowireBean(classToAutowire);
}
}
}
}

If you are inside of Spring bean (in this case #Controller bean) you shouldn't use Spring context instance at all. Just autowire className bean directly.
BTW, avoid using field injection as it's considered as bad practice.

One API method I use when I'm not sure what the bean name is org.springframework.beans.factory.ListableBeanFactory#getBeanNamesForType(java.lang.Class<?>). I simple pass it the class type and it retrieves a list of beans for me. You can be as specific or general as you'd like to retrieve all the beans associated with that type and its subtypes, example
#Autowired
ApplicationContext ctx
...
SomeController controller = ctx.getBeanNamesForType(SomeController)

Easy way in configration class call the BEAN annoted method . Yes u heard it right---- :P calling SpringBoot #Bean annoted method return the same bean from config .I was trying to call a logout in #predestroy method in config class from a bean and direcltly called the method to get the same bean .
P.S. : I added debug in the #bean annotated method but it didn't entered the method even when i called it.Sure to blame -----> Spring Magic <----

You can use ServiceLocatorFactoryBean. First you need to create an interface for your class
public interface YourClassFactory {
YourClass getClassByName(String name);
}
Then you have to create a config file for ServiceLocatorBean
#Configuration
#Component
public class ServiceLocatorFactoryBeanConfig {
#Bean
public ServiceLocatorFactoryBean serviceLocatorBean(){
ServiceLocatorFactoryBean bean = new ServiceLocatorFactoryBean();
bean.setServiceLocatorInterface(YourClassFactory.class);
return bean;
}
}
Now you can find your class by name like that
#Autowired
private YourClassfactory factory;
YourClass getYourClass(String name){
return factory.getClassByName(name);
}

Just use:
org.springframework.beans.factory.BeanFactory#getBean(java.lang.Class)
Example:
#Component
public class Example {
#Autowired
private ApplicationContext context;
public MyService getMyServiceBean() {
return context.getBean(MyService.class);
}
// your code uses getMyServiceBean()
}