I'm trying to understand if I can combine reflection with spring dependency injection as the following:
public interface ClientCommand {
public void execute(...);
public static enum Command {
SomeCommand(SomeCommand.class);
private Class<? extends ClientCommand> clazz;
private Command(Class<? extends ClientCommand> clazz) {
this.clazz = clazz;
}
public Class<? extends ClientCommand> getClazz() {
return clazz;
}
public static ClientCommand getClientCommand(String name) {
Command command = Enum.valueOf(Command.class, name);
return command.getClazz().newInstance();
}
}
}
This will create an instance of a command class based on the name passed in getClientCommand.
This is an example of class extending ClientCommand:
public class LoginCommand implements ClientCommand {
#Autowired
private UserRepository userRepository;
public void setUserRepository(#Qualifier("userRepository")UserRepository userRepository) {
this.userRepository = userRepository;
}
public void execute(...) {
...
}
}
And the repository is something like:
#Repository("userRepository")
public class UserRepositoryImpl implements UserRepository {
....
}
When the LoginCommand.execute() method is executed, the UserRepository is null.
If I use the newInstance() to create the object, does spring care at all to inject the dependencies?
More than for practical use, is to understand if is theoretically possible to get this code working.
Thanks in advance
To answer this question:
If I use the newInstance() to create the object, does spring care at all to inject the dependencies?
I will answer with no, not by default. Spring will only inject dependencies on objects that Spring is in control of, and if you are using reflection to instantiate it, or the new operator, then you are the one in control, not Spring.
But, there is hope. You can use AspectJ to do bytecode weaving when the new operator is used, or even when Class.newInstance() is used.
Take a look at this Spring documentation for more on this approach.
Since you're creating the object on your own Spring will not do dependency injection on the object. It will also not add any AOP proxies for it if its configured to do that.
You can either use AspectJ to instrument your code by adding the logic necessary to do dependency injection on the instance. This is done completely transparently.
Or you can do it yourself by using AutowireCapableBeanFactory. It's a semi-internal interface that you can use and its intended for just this purpose. It has a set of methods that do various parts of creating and injecting, you'll probably need the createBean() method.
You can get an AutowireCapableBeanFactory by calling getAutowireCapableBeanFactory on your ApplicationContext.
In your case it would probably be a good idea to create a CommandFactory, make that implement ApplicationContextAware and have a method like createCommand() that calls createBean().
Related
I'm trying to make a custom validation (checking if an email is already present in the database). For single class my annotation is working fine but I need to make this validation work for two objects implementing common interface. I have User interface and Visitor and Exhibitor classes which are implementing it.
Here is my annotation:
#Documented
#Constraint(validatedBy = UniqueEmailValidator.class)
#Target({ElementType.METHOD, ElementType.FIELD})
#Retention(RetentionPolicy.RUNTIME)
public #interface UniqueEmail {
String message() default "Email is already existing!";
Class<?>[] groups() default {};
Class<? extends Payload>[] payload() default {};
}
Here is Validator class
public class UniqueEmailValidator implements ConstraintValidator<UniqueEmail, String> {
#Autowired
private ApplicationContext applicationContext;
private UserService userService;
#Override
public void initialize(UniqueEmail uniqueEmail) {
}
#Override
public boolean isValid(String email, ConstraintValidatorContext constraintValidatorContext) {
return !userService.isEmailPresent(email);
}
}
UserService is a common interface of VisitorService and ExhibitorService
public interface UserService {
boolean isEmailPresent(String email);
}
And it's implementation...
#Service
public class VisitorService implements UserService {
#Autowired
VisitorDao visitorDao;
#Override
public boolean isEmailPresent(String email) {
try {
return !visitorDao.findAllByEmail(email).isEmpty();
} catch (NullPointerException e) {
return false;
}
}
}
Currently I'm getting NullPointerException
java.lang.NullPointerException: null
at pl.com.sremski.testapp.validators.UniqueEmailValidator.isValid(UniqueEmailValidator.java:24) ~[classes/:na]
at pl.com.sremski.testapp.validators.UniqueEmailValidator.isValid(UniqueEmailValidator.java:10)
Any ideas what's the reason? I was trying to debug and UserService is null... but I'm trying to add a new visitor so it should use VisitorService. Please help.
I managed to solve my problem by specifing exact implementation in annotation's parameter
#UniqueEmail(service = VisitorService.class)
and then creating it's instance in
#Override
public void initialize(UniqueEmail uniqueEmail) {
}
However I had to separate all Spring-based validation from Hibernate's entity to make it work.
If you don't want to have two different annotations with different #Qualifier() marked beans, you can consider to choose that bean in runtime using application context. But injecting whole Spring context to your business logic is considered as a very bad practice:
1) Mixing infrastructure (your WHOLE infrastructure) with business processes makes your code hard to understand and decouple.
2) It is hard to unit test this, need to mock the context object instead of your services.
But after using Google Guice DI I found myself in using Provider<Service> pattern, because injecting spring into spring is OK. So you can create a class like:
#Service
class UserServiceProvider<T extends UserService> implements Provider<T> {
#Autowired private ApplicationContext context;
public UserService get(Class<T> exactServiceType) {
return (UserService) context.getBean(exactServiceType);
}
}
Maybe there is a much better "Spring way" to do this, but this code is easy to understand and maintain. Works lile a Scope.Prototype bean, but a bit more flexible.
Google Guice Provider tutorial
EDIT:
Spring has a similar interface FactoryBean<T> and it has an enhanced handling, because DI will inject not the factory, but what factory provides.
But the one problem is you can't do it with some condition.
Simple example
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.
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;
}
}
I'd like to provide a default bean by a custom jar. Only if the user implements a specific abstract class the default bean injection should be skipped.
The following setup already works fine, except one thing: any injected classes within the default wired class are null! What might I be missing?
#Configration
public class AppConfig {
//use the default service if the user does not provide an own implementation
#Bean
#Conditional(MissingServiceBean.class)
public MyService myService() {
return new MyService() {};
}
}
#Component
public abstract class MyService {
#Autowired
private SomeOtherService other;
//default impl of the method, that may be overridden
public void run() {
System.out.println(other); //null! Why?
}
}
public class MissingServiceBean implements Condition {
#Override
public boolean matches(ConditionContext context, AnnotatedTypeMetadata metadata) {
return context.getBeanFactory().getBeansOfType(MyService.class).isEmpty();
}
}
The MyService bean is created and can also be injected. But contained classes are null.
If I remove the #Conditioanl annotation everything works as expected.
Your simplest possibility is the usage of the #Primary annotation. You define your interface/abstract class and build a default implementation. Until here thats the basic spring autowiring.
Now you create another implementation with #Primary and make it available in the application context. Spring will now pick up the primary implementation for the autowiring.
Another possibilty in Spring 4.1+ would be to autowire an ordered List<Intf> and ask the interface with a supports(...) call to fetch the current implementation for whatever parameter you give into supports. You give the default implementation a low priority and the more detailed ones a higher priority. Like this you can even build a more detailed default behavior. I'm using this approach for several configurations to handle different classes with default and specific implementations.
One example would be during permission evaluation where we have a default config for the base classes, another higher one for domain classes, and a even higher possible one for specific domain entities. The permission evaluator goes through the list and checks each implementation if it supports that class and delegates to the implementation in that case.
I dont have the code here but i could share it later if desired to make that more clear.
Change your code to the following:
public abstract class MyService {
private final SomeOtherService other;
public MyService(SomeOtherService other) {
this.other = other;
}
//default impl of the method, that may be overridden
public void run() {
System.out.println(other);
}
}
#Configration
public class AppConfig {
#Autowired
private SomeOtherService other;
//use the default service if the user does not provide an own implementation
#Bean
#Condition(MissingServiceBean.class)
public MyService myService() {
return new MyService(other) {};
}
}
I stumbled upon a case where the AOP proxy created by using #Cacheable breaks the dependency injection in Spring 3.1.1. Here is my scenario:
I have an interface and a class implementing this interface using #Cacheable at the implemented method.
Example interface:
public interface ImgService {
public byte[] getImage(String name);
}
Example implementation:
public class ImgServiceImpl implements ImgService {
#Cacheable(cacheName = "someCache")
public byte[] getImage(String name){//TODO};
protected String someOtherMethod(){//};
}
I also have to JUnit test classes - one which injects the interface and one the implementation:
#RunWith(SpringJUnit4ClassRunner.class)
#ContextConfiguration(locations = { "classpath*:META-INF/spring.xml" })
public class ImgServiceTest {
#Inject
private ImgService;
}
and
#RunWith(SpringJUnit4ClassRunner.class)
#ContextConfiguration(locations = { "classpath*:META-INF/spring.xml" })
public class ImgServiceImplTest {
#Inject
private ImgServiceImpl;
}
Dependency injection for the interface works fine. However, when I get to injecting the implementation in the second test class I get an "Injection of autowired dependencies failed". I was able to debug it and it appears that ClassUtils.isAssignableValue() wrongly compares the desired type to the proxy class. It is called by DefaultListableBeanFactory. What is even stranger is that if I remove the #Cacheable annotation from the implemented method and add it to some other protected/private method, dependency injection works fine again. Is this a bug and what would be the correct approach to handle this situation?
It's not a bug, it's an expected side-effect of using JDK dynamic proxies for AOP implementation.
Since all calls to the cacheable method of ImgServiceImpl should go through the dynamic proxy of type ImgService, there is no way to inject this dependency into a field of type ImgServiceImpl.
When you move #Cacheable to private or protected method, injection works because #Cacheable doesn't take effect in this case - only public methods can be adviced using proxy-based AOP.
So, you should either declare fields to be injected as ImgService, or configure Spring to use target class-based proxies instead using proxy-target-class = "true".
Yet another option is to configure Spring to use AspectJ-based AOP implementation (requires compile-time or load-time weaving).
It's applicable to all AOP-based features provided by Spring (transactions, security, async execution, cache, custom aspects, etc).
See also:
7.6 Proxying mechanisms
OK, so here is the solution I came up finally. I implemented a simple method that attempts to extract the target object from the proxy based on its implementation of the org.springframework.aop.framework.Advised class:
#SuppressWarnings({"unchecked"})
public static <T> T getTargetObject(Object proxy, Class<T> targetClass) {
if (AopUtils.isJdkDynamicProxy(proxy)) {
try {
return (T) ((Advised)proxy).getTargetSource().getTarget();
} catch (Exception e) {
return null;
}
} else {
return (T) proxy;
}
}
My implementation test class now looks like this:
#RunWith(SpringJUnit4ClassRunner.class)
#ContextConfiguration(locations = { "classpath*:META-INF/spring.xml" })
public class ImgServiceImplTest {
#Inject
private ImgService imgService;
private ImgServiceImpl imgServiceImpl;
#PostConstruct
public void setUp() {
imgServiceImpl = getTargetObject(imgService, ImgServiceImpl.class);
}
}