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Why is my Spring #Autowired field null?
(21 answers)
Closed 1 year ago.
Hello, I've been trying to use a service into another one but I get null. Does anyone know how can I inject it correctly?
(My ClassServiceB has its #Service annotation)
This is my service class...
#Service
public class ClassServiceAImpl implements ClassServiceA {
#Autowired
ClassServiceB classServiceB;
#Override
public ClassOutDto searchDriverForOrder(InputDto inputDto ) {
classServiceB.doSomething(); //In this part classServiceB is getting null
...
}
Did you try to use #Autowired in the constructor class? It is easier when you want to do unit tests and the recommendation to do injection depended. Would be like this:
#Service
public class ClassServiceAImpl implements ClassServiceA {
private ClassServiceB classServiceB;
#Autowired
public ClassServiceAImpl(ClassServiceB classServiceB) {
this.classServiceB = classServiceB;
}
#Override
public ClassOutDto searchDriverForOrder(InputDto inputDto ) {
classServiceB.doSomething(); //In this part classServiceB is getting null
...
}
}
Don't forget that your ClassServiceB must also be annotated with #Service. Your interface should not be rated. If that doesn't help, share more of the properties, interface and ClassServiceB.
I have a service which needs to create Agents on the runtime. Agents inherit from a base Agent class. I would like to use the Autowired ability of spring instead of doing my own dependency injections.
But I am running into this issue, even though I am marking the component as scope=prototype, and even #Lazy to prevent anything from happening at compile-time.
***************************
APPLICATION FAILED TO START
***************************
Description:
Parameter 0 of constructor in com.my.project.AgentType1 required a bean of type 'com.my.project.POJO' that could not be found.
This is the service that tries to create the agents:
#Service
public class ProjectMain {
#Autowired
ApplicationContext context;
List<IAgent> agents = new ArrayList<>();
void SetupAgents(List<POJO> agentPojos) {
for(POJO agentPojo: agentPojos) {
IAgent agent = AgentFactory.CreateAgent(agentPojo, context);
agents.add(agent);
}
}
}
This is the factory class, not marked as #Component etc. It uses the context passed to it to create the child class beans. It tries to pass the constructor argument via the getBean method.
public class AgentFactory {
public static IAgent CreateAgent(POJO agentPojo, ApplicationContext context) {
if (agentPojo.type.equals("AgentType1")) {
return context.getBean(AgentType1.class, agentPojo);
} else {
return context.getBean(AgentType2.class, agentPojo);
}
}
}
This is a custom annotation which I found is needed for inheritance scenarios.
#Target({ ElementType.TYPE })
#Retention(RetentionPolicy.RUNTIME)
#Component
#Inherited
#Lazy
#Scope("prototype")
public #interface AgentAnnotation {}
These are the base and child agent classes, which need a custom data structure called POJO to work.
#AgentAnnotation
public class BaseAgent implements IAgent {
#Autowired
Environment env;
public BaseAgent(POJO agentPojo, String someotherdata) {
}
}
public class AgentType1 extends BaseAgent {
public AgentType1(POJO agentPojo) {
super(agentPojo, "mydata1");
...
}
}
public class AgentType2 extends BaseAgent {
public AgentType2(POJO agentPojo) {
super(agentPojo, "mydata2");
...
}
}
This is the starter app.
#ComponentScan(basePackages = "com.my.project", includeFilters = #ComponentScan.Filter(AgentAnnotation.class))
#EnableScheduling
#SpringBootApplication
public class MyApplication {
public static void main(String[] args) {
SpringApplication.run(MyApplication.class, args);
}
}
I also tried the configuration approach:
#Configuration
public class BaseAgentConfig {
#Bean
#Scope("prototype")
public AgentType1 agentType1(POJO agentPojo) {
return new AgentType1(agentPojo);
}
#Bean
#Scope("prototype")
public AgentType2 agentType2(POJO agentPojo) {
return new AgentType2(agentPojo);
}
}
In this case, I removed the #AgentAnnotation from the baseAgent class as we are now instantiating through this config. Also removed the ComponentScan line from the main App.
This time around, the #Autowired doesn't work. All Autowired references in the baseAgent class are null.
Please advise on the best approach to solve this error. Thanks.
Found the issue and solution.
Basically, I was expecting child classes to inherit #Component and #Scope, which it doesn't.
So essentially, I need to annotate each child class with #Component and #Scope("prototype").
The other problem was that I was expecting Autowired items in the constructor, which was too early. Adding a #PostConstruct addressed that issue.
So I ended up deleting the custom annotation and the configuration class and making the changes I just described.
I am trying to use #Async annotation provided by spring. Going through some of the blogs I found there are the following constraints for using it:
It must be applied to public methods only
Self-invocation – calling the async method from within the same class – won’t work
I have a method which is getting called from the same class which I want to mark #Async. Is there any way of achieving it from the same class?
In Spring v4.3+ you can use self injection, and call the method on the self injected reference.
So for example:
#Component
public class SomeClass {
#Autowired
private SomeClass selfInjected;
public void someMethod() {
selfInjected.someOtherMethod();
}
#Async
public void someOtherMethod(){
...;
}
}
Updated as OP is using version before 4.3:
This will work for you.
#Component
public class SomeClass {
#Autowired
private ApplicationContext applicationContext;
private SomeClass selfInjected;
#PostConstruct
private void init() {
selfInjected = applicationContext.getBean(SomeClass.class);
}
}
Or
The other option is to extract the method to separate class and autowire it. I would personally explore this option before doing the above method.
I am trying to create a Component that will be Autowired unless the user creates a different implementation.
I used the following code to try and isolate the problem:
The interface:
public interface A {...}
The implementation:
#Component
#ConditionalOnMissingBean(A.class)
public class AImpl implements A {...}
The usage code:
public class AUsage {
#Autowired
private A a;
}
In this example, I don't get AImpl autowired into AUsage.
If I implement A in another class without the ConditionalOnMissingBean it works.
I tried copying existing uses of #ConditionalOnMissingBean from the internet and noticed that they all reference a #Bean method.
Indeed, when I added this code to AUsage:
public class AUsage {
#Autowired
private A a;
#Bean
#ConditionalOnMissingBean
public A createA() {
return new AImpl();
}
}
and removed the annotations from AImpl:
public class AImpl implements A {...}
everything works as expected.
I'd be pleased to get an explanation to this, if anyone knows.
In a java-spring web-app I would like to be able to dynamically inject beans.
For example I have an interface with 2 different implementations:
In my app I'm using some properties file to configure injections:
#Determines the interface type the app uses. Possible values: implA, implB
myinterface.type=implA
My injections actually loaded conditionally relaying on the properties values in the properties file. For example in this case myinterface.type=implA wherever I inject MyInterface the implementation that will be injected will be ImplA (I accomplished that by extending the Conditional annotation).
I would like that during runtime - once the properties are changed the following will happen (without server restart):
The right implementation will be injected. For example when setting myinterface.type=implB ImplB will be injected where-ever MyInterface is used
Spring Environment should be refreshed with the new values and re-injected as well to beans.
I thought of refreshing my context but that creates problems.
I thought maybe to use setters for injection and re-use those setters once properties are re-configured. Is there a working practice for such a requirement?
Any ideas?
UPDATE
As some suggested I can use a factory/registry that holds both implementations (ImplA and ImplB) and returns the right one by querying the relevant property.
If I do that I still have the second challenge - the environment. for example if my registry looks like this:
#Service
public class MyRegistry {
private String configurationValue;
private final MyInterface implA;
private final MyInterface implB;
#Inject
public MyRegistry(Environmant env, MyInterface implA, MyInterface ImplB) {
this.implA = implA;
this.implB = implB;
this.configurationValue = env.getProperty("myinterface.type");
}
public MyInterface getMyInterface() {
switch(configurationValue) {
case "implA":
return implA;
case "implB":
return implB;
}
}
}
Once property has changed I should re-inject my environment. any suggestions for that?
I know I can query that env inside the method instead of constructor but this is a performance reduction and also I would like to think of an ider for re-injecting environment (again, maybe using a setter injection?).
I would keep this task as simple as possible. Instead of conditionally load one implementation of the MyInterface interface at startup and then fire an event that triggers dynamic loading of another implementation of the same interface, I would tackle this problem in a different way, that is much simpler to implement and maintain.
First of all, I'd just load all possible implementations:
#Component
public class MyInterfaceImplementationsHolder {
#Autowired
private Map<String, MyInterface> implementations;
public MyInterface get(String impl) {
return this.implementations.get(impl);
}
}
This bean is just a holder for all implementations of the MyInterface interface. Nothing magic here, just common Spring autowiring behavior.
Now, wherever you need to inject a specific implementation of MyInterface, you could do it with the help of an interface:
public interface MyInterfaceReloader {
void changeImplementation(MyInterface impl);
}
Then, for every class that needs to be notified of a change of the implementation, just make it implement the MyInterfaceReloader interface. For instance:
#Component
public class SomeBean implements MyInterfaceReloader {
// Do not autowire
private MyInterface myInterface;
#Override
public void changeImplementation(MyInterface impl) {
this.myInterface = impl;
}
}
Finally, you need a bean that actually changes the implementation in every bean that has MyInterface as an attribute:
#Component
public class MyInterfaceImplementationUpdater {
#Autowired
private Map<String, MyInterfaceReloader> reloaders;
#Autowired
private MyInterfaceImplementationsHolder holder;
public void updateImplementations(String implBeanName) {
this.reloaders.forEach((k, v) ->
v.changeImplementation(this.holder.get(implBeanName)));
}
}
This simply autowires all beans that implement the MyInterfaceReloader interface and updates each one of them with the new implementation, which is retrieved from the holder and passed as an argument. Again, common Spring autowiring rules.
Whenever you want the implementation to be changed, you should just invoke the updateImplementations method with the name of the bean of the new implementation, which is the lower camel case simple name of the class, i.e. myImplA or myImplB for classes MyImplA and MyImplB.
You should also invoke this method at startup, so that an initial implementation is set on every bean that implements the MyInterfaceReloader interface.
I solved a similar issue by using org.apache.commons.configuration.PropertiesConfiguration and org.springframework.beans.factory.config.ServiceLocatorFactoryBean:
Let VehicleRepairService be an interface:
public interface VehicleRepairService {
void repair();
}
and CarRepairService and TruckRepairService two classes that implements it:
public class CarRepairService implements VehicleRepairService {
#Override
public void repair() {
System.out.println("repair a car");
}
}
public class TruckRepairService implements VehicleRepairService {
#Override
public void repair() {
System.out.println("repair a truck");
}
}
I create an interface for a service factory:
public interface VehicleRepairServiceFactory {
VehicleRepairService getRepairService(String serviceType);
}
Let use Config as configuration class:
#Configuration()
#ComponentScan(basePackages = "config.test")
public class Config {
#Bean
public PropertiesConfiguration configuration(){
try {
PropertiesConfiguration configuration = new PropertiesConfiguration("example.properties");
configuration
.setReloadingStrategy(new FileChangedReloadingStrategy());
return configuration;
} catch (ConfigurationException e) {
throw new IllegalStateException(e);
}
}
#Bean
public ServiceLocatorFactoryBean serviceLocatorFactoryBean() {
ServiceLocatorFactoryBean serviceLocatorFactoryBean = new ServiceLocatorFactoryBean();
serviceLocatorFactoryBean
.setServiceLocatorInterface(VehicleRepairServiceFactory.class);
return serviceLocatorFactoryBean;
}
#Bean
public CarRepairService carRepairService() {
return new CarRepairService();
}
#Bean
public TruckRepairService truckRepairService() {
return new TruckRepairService();
}
#Bean
public SomeService someService(){
return new SomeService();
}
}
By using FileChangedReloadingStrategy your configuration be reload when you change the property file.
service=truckRepairService
#service=carRepairService
Having the configuration and the factory in your service, let you can get the appropriate service from the factory using the current value of the property.
#Service
public class SomeService {
#Autowired
private VehicleRepairServiceFactory factory;
#Autowired
private PropertiesConfiguration configuration;
public void doSomething() {
String service = configuration.getString("service");
VehicleRepairService vehicleRepairService = factory.getRepairService(service);
vehicleRepairService.repair();
}
}
Hope it helps.
If I understand you correctly then the goal is not to replace injected object instances but to use different implementations during interface method call depends on some condition at run time.
If it is so then you can try to look at the Sring TargetSource mechanism in combination with ProxyFactoryBean. The point is that proxy objects will be injected to beans that uses your interface, and all the interface method calls will be sent to TargetSource target.
Let's call this "Polymorphic Proxy".
Have a look at example below:
ConditionalTargetSource.java
#Component
public class ConditionalTargetSource implements TargetSource {
#Autowired
private MyRegistry registry;
#Override
public Class<?> getTargetClass() {
return MyInterface.class;
}
#Override
public boolean isStatic() {
return false;
}
#Override
public Object getTarget() throws Exception {
return registry.getMyInterface();
}
#Override
public void releaseTarget(Object target) throws Exception {
//Do some staff here if you want to release something related to interface instances that was created with MyRegistry.
}
}
applicationContext.xml
<bean id="myInterfaceFactoryBean" class="org.springframework.aop.framework.ProxyFactoryBean">
<property name="proxyInterfaces" value="MyInterface"/>
<property name="targetSource" ref="conditionalTargetSource"/>
</bean>
<bean name="conditionalTargetSource" class="ConditionalTargetSource"/>
SomeService.java
#Service
public class SomeService {
#Autowired
private MyInterface myInterfaceBean;
public void foo(){
//Here we have `myInterfaceBean` proxy that will do `conditionalTargetSource.getTarget().bar()`
myInterfaceBean.bar();
}
}
Also if you want to have both MyInterface implementations to be Spring beans, and the Spring context could not contains both instances at the same time then you can try to use ServiceLocatorFactoryBean with prototype target beans scope and Conditional annotation on target implementation classes. This approach can be used instead of MyRegistry.
P.S.
Probably Application Context refresh operation also can do what you want but it can cause other problems such as performance overheads.
This may be a duplicate question or at least very similar, anyway I answered this sort of question here: Spring bean partial autowire prototype constructor
Pretty much when you want a different beans for a dependency at run-time you need to use a prototype scope. Then you can use a configuration to return different implementations of the prototype bean. You will need to handle the logic on which implementation to return yourself, (they could even be returning 2 different singleton beans it doesn't matter) But say you want new beans, and the logic for returning the implementation is in a bean called SomeBeanWithLogic.isSomeBooleanExpression(), then you can make a configuration:
#Configuration
public class SpringConfiguration
{
#Bean
#Autowired
#Scope("prototype")
public MyInterface createBean(SomeBeanWithLogic someBeanWithLogic )
{
if (someBeanWithLogic .isSomeBooleanExpression())
{
return new ImplA(); // I could be a singleton bean
}
else
{
return new ImplB(); // I could also be a singleton bean
}
}
}
There should never be a need to reload the context. If for instance, you want the implementation of a bean to change at run-time, use the above. If you really need to reload your application, because this bean was used in constructors of a singleton bean or something weird, then you need to re-think your design, and if these beans are really singleton beans. You shouldn't be reloading the context to re-create singleton beans to achieve different run-time behavior, that is not needed.
Edit The first part of this answer answered the question about dynamically injecting beans. As asked, but I think the question is more of one: 'how can I change the implementation of a singleton bean at run-time'. This could be done with a proxy design pattern.
interface MyInterface
{
public String doStuff();
}
#Component
public class Bean implements MyInterface
{
boolean todo = false; // change me as needed
// autowire implementations or create instances within this class as needed
#Qualifier("implA")
#Autowired
MyInterface implA;
#Qualifier("implB")
#Autowired
MyInterface implB;
public String doStuff()
{
if (todo)
{
return implA.doStuff();
}
else
{
return implB.doStuff();
}
}
}
You can use #Resource annotation for injection as originally answered here
e.g.
#Component("implA")
public class ImplA implements MyInterface {
...
}
#Component("implB")
public class ImplB implements MyInterface {
...
}
#Component
public class DependentClass {
#Resource(name = "\${myinterface.type}")
private MyInterface impl;
}
and then set the implementation type in properties file as -
myinterface.type=implA
Be aware that - if interesting to know about - FileChangedReloadingStrategy makes your project highly dependent on the deployment conditions: the WAR/EAR should be exploded by container and your should have direct access to the file system, conditions that are not always met in all situations and environments.
You can use Spring #Conditional on a property value. Give both Beans the same name and it should work as only one Instance will be created.
Have a look here on how to use #Conditional on Services and Components:
http://blog.codeleak.pl/2015/11/how-to-register-components-using.html
public abstract class SystemService {
}
public class FooSystemService extends FileSystemService {
}
public class GoSystemService extends FileSystemService {
}
#Configuration
public class SystemServiceConf {
#Bean
#Conditional(SystemServiceCondition.class)
public SystemService systemService(#Value("${value.key}") value) {
switch (value) {
case A:
return new FooSystemService();
case B:
return new GoSystemService();
default:
throw new RuntimeException("unknown value ");
}
}
}
public class SystemServiceCondition implements Condition {
#Override
public boolean matches(ConditionContext conditionContext, AnnotatedTypeMetadata annotatedTypeMetadata) {
return true;
}
}