I've switched a portion of a Spring app to use functional bean registrations. The motivation for the switch is due to requiring multiple instances of some beans under certain conditions. It also turns out to be much more concise (which won't be at all apparent with the simplistic examples below).
The code used to look like this (simple example):
#Configuration
public class ConfigA {
#Bean
public BeanA beanA() {
return new BeanA();
}
}
#Service
public class Service1 {
#Autowired BeanA beanA;
...
}
#Service
public class Service2 {
#Autowired BeanA beanA;
...
}
I've switched the configuration class to look like this:
#Configuration
public class ConfigA implements ApplicationContextInitializer<GenericApplicationContext> {
#Override
public void initialize(GenericApplicationContext context) {
context.registerBean("beanA", BeanA.class, () -> new BeanA());
}
}
The issue I'm now encountering is that Spring is complaining about autowired beans not being found. With the original code, Spring could determine that a BeanA bean was declared via ConfigA and would create that bean before initializing the services.
With the new code, I guess there is no way for Spring to determine where the BeanA bean(s) are being declared, and so it tries to init the services before the BeanA is initialized (which causes the app to not start).
I was hoping that Spring would prioritize #Configuration classes over #Service or #Controller classes, but that doesn't seem to be the case.
I could annotate all the services with #DependsOn("configA"), but there are many services that autowire BeanA (some in other code bases), so the #DependsOn option isn't really realistic.
Question: When using functional bean registration, is there a way to inform Spring that ConfigA is responsible for creating an instance of BeanA?
In order to use the functional style of bean registration and enable autowiring mechanism in other beans you can do the following:
Remove #Configuration annotation from your ConfigA class;
Create directory named META-INF under main/java/resources and create a file named spring.factories under the newly created directory;
Fill the newly created file with the line org.springframework.context.ApplicationContextInitializer=(package-name-to-configA-class).ConfigA
Now Spring should be able to successfully autowire bean named BeanA where requested.
Related
Please explain the following about NoSuchBeanDefinitionException exception in Spring:
What does it mean?
Under what conditions will it be thrown?
How can I prevent it?
This post is designed to be a comprehensive Q&A about occurrences of NoSuchBeanDefinitionException in applications using Spring.
The javadoc of NoSuchBeanDefinitionException explains
Exception thrown when a BeanFactory is asked for a bean instance for
which it cannot find a definition. This may point to a non-existing
bean, a non-unique bean, or a manually registered singleton instance
without an associated bean definition.
A BeanFactory is basically the abstraction representing Spring's Inversion of Control container. It exposes beans internally and externally, to your application. When it cannot find or retrieve these beans, it throws a NoSuchBeanDefinitionException.
Below are simple reasons why a BeanFactory (or related classes) would not be able to find a bean and how you can make sure it does.
The bean doesn't exist, it wasn't registered
In the example below
#Configuration
public class Example {
public static void main(String[] args) throws Exception {
AnnotationConfigApplicationContext ctx = new AnnotationConfigApplicationContext(Example.class);
ctx.getBean(Foo.class);
}
}
class Foo {}
we haven't registered a bean definition for the type Foo either through a #Bean method, #Component scanning, an XML definition, or any other way. The BeanFactory managed by the AnnotationConfigApplicationContext therefore has no indication of where to get the bean requested by getBean(Foo.class). The snippet above throws
Exception in thread "main" org.springframework.beans.factory.NoSuchBeanDefinitionException:
No qualifying bean of type [com.example.Foo] is defined
Similarly, the exception could have been thrown while trying to satisfy an #Autowired dependency. For example,
#Configuration
#ComponentScan
public class Example {
public static void main(String[] args) throws Exception {
AnnotationConfigApplicationContext ctx = new AnnotationConfigApplicationContext(Example.class);
}
}
#Component
class Foo { #Autowired Bar bar; }
class Bar { }
Here, a bean definition is registered for Foo through #ComponentScan. But Spring knows nothing of Bar. It therefore fails to find a corresponding bean while trying to autowire the bar field of the Foo bean instance. It throws (nested inside a UnsatisfiedDependencyException)
Caused by: org.springframework.beans.factory.NoSuchBeanDefinitionException:
No qualifying bean of type [com.example.Bar] found for dependency [com.example.Bar]:
expected at least 1 bean which qualifies as autowire candidate for this dependency. Dependency annotations: {#org.springframework.beans.factory.annotation.Autowired(required=true)}
There are multiple ways to register bean definitions.
#Bean method in a #Configuration class or <bean> in XML configuration
#Component (and its meta-annotations, eg. #Repository) through #ComponentScan or <context:component-scan ... /> in XML
Manually through GenericApplicationContext#registerBeanDefinition
Manually through BeanDefinitionRegistryPostProcessor
...and more.
Make sure the beans you expect are properly registered.
A common error is to register beans multiple times, ie. mixing the options above for the same type. For example, I might have
#Component
public class Foo {}
and an XML configuration with
<context:component-scan base-packages="com.example" />
<bean name="eg-different-name" class="com.example.Foo />
Such a configuration would register two beans of type Foo, one with name foo and another with name eg-different-name. Make sure you're not accidentally registering more beans than you wanted. Which leads us to...
If you're using both XML and annotation-based configurations, make sure you import one from the other. XML provides
<import resource=""/>
while Java provides the #ImportResource annotation.
Expected single matching bean, but found 2 (or more)
There are times when you need multiple beans for the same type (or interface). For example, your application may use two databases, a MySQL instance and an Oracle one. In such a case, you'd have two DataSource beans to manage connections to each one. For (simplified) example, the following
#Configuration
public class Example {
public static void main(String[] args) throws Exception {
AnnotationConfigApplicationContext ctx = new AnnotationConfigApplicationContext(Example.class);
System.out.println(ctx.getBean(DataSource.class));
}
#Bean(name = "mysql")
public DataSource mysql() { return new MySQL(); }
#Bean(name = "oracle")
public DataSource oracle() { return new Oracle(); }
}
interface DataSource{}
class MySQL implements DataSource {}
class Oracle implements DataSource {}
throws
Exception in thread "main" org.springframework.beans.factory.NoUniqueBeanDefinitionException:
No qualifying bean of type [com.example.DataSource] is defined:
expected single matching bean but found 2: oracle,mysql
because both beans registered through #Bean methods satisfied the requirement of BeanFactory#getBean(Class), ie. they both implement DataSource. In this example, Spring has no mechanism to differentiate or prioritize between the two. But such mechanisms exists.
You could use #Primary (and its equivalent in XML) as described in the documentation and in this post. With this change
#Bean(name = "mysql")
#Primary
public DataSource mysql() { return new MySQL(); }
the previous snippet would not throw the exception and would instead return the mysql bean.
You can also use #Qualifier (and its equivalent in XML) to have more control over the bean selection process, as described in the documentation. While #Autowired is primarily used to autowire by type, #Qualifier lets you autowire by name. For example,
#Bean(name = "mysql")
#Qualifier(value = "main")
public DataSource mysql() { return new MySQL(); }
could now be injected as
#Qualifier("main") // or #Qualifier("mysql"), to use the bean name
private DataSource dataSource;
without issue. #Resource is also an option.
Using wrong bean name
Just as there are multiple ways to register beans, there are also multiple ways to name them.
#Bean has name
The name of this bean, or if plural, aliases for this bean. If left
unspecified the name of the bean is the name of the annotated method.
If specified, the method name is ignored.
<bean> has the id attribute to represent the unique identifier for a bean and name can be used to create one or more aliases illegal in an (XML) id.
#Component and its meta annotations have value
The value may indicate a suggestion for a logical component name, to
be turned into a Spring bean in case of an autodetected component.
If that's left unspecified, a bean name is automatically generated for the annotated type, typically the lower camel case version of the type name. For example MyClassName becomes myClassName as its bean name. Bean names are case sensitive. Also note that wrong names/capitalization typically occur in beans referred to by string like #DependsOn("my BeanName") or XML config files.
#Qualifier, as mentioned earlier, lets you add more aliases to a bean.
Make sure you use the right name when referring to a bean.
More advanced cases
Profiles
Bean definition profiles allow you to register beans conditionally. #Profile, specifically,
Indicates that a component is eligible for registration when one or
more specified profiles are active.
A profile is a named logical grouping that may be activated
programmatically via
ConfigurableEnvironment.setActiveProfiles(java.lang.String...) or
declaratively by setting the spring.profiles.active property as a JVM
system property, as an environment variable, or as a Servlet context
parameter in web.xml for web applications. Profiles may also be
activated declaratively in integration tests via the #ActiveProfiles
annotation.
Consider this examples where the spring.profiles.active property is not set.
#Configuration
#ComponentScan
public class Example {
public static void main(String[] args) throws Exception {
AnnotationConfigApplicationContext ctx = new AnnotationConfigApplicationContext(Example.class);
System.out.println(Arrays.toString(ctx.getEnvironment().getActiveProfiles()));
System.out.println(ctx.getBean(Foo.class));
}
}
#Profile(value = "StackOverflow")
#Component
class Foo {
}
This will show no active profiles and throw a NoSuchBeanDefinitionException for a Foo bean. Since the StackOverflow profile wasn't active, the bean wasn't registered.
Instead, if I initialize the ApplicationContext while registering the appropriate profile
AnnotationConfigApplicationContext ctx = new AnnotationConfigApplicationContext();
ctx.getEnvironment().setActiveProfiles("StackOverflow");
ctx.register(Example.class);
ctx.refresh();
the bean is registered and can be returned/injected.
AOP Proxies
Spring uses AOP proxies a lot to implement advanced behavior. Some examples include:
Transaction management with #Transactional
Caching with #Cacheable
Scheduling and asynchronous execution with #Async and #Scheduled
To achieve this, Spring has two options:
Use the JDK's Proxy class to create an instance of a dynamic class at runtime which only implements your bean's interfaces and delegates all method invocations to an actual bean instance.
Use CGLIB proxies to create an instance of a dynamic class at runtime which implements both interfaces and concrete types of your target bean and delegates all method invocations to an actual bean instance.
Take this example of JDK proxies (achieved through #EnableAsync's default proxyTargetClass of false)
#Configuration
#EnableAsync
public class Example {
public static void main(String[] args) throws Exception {
AnnotationConfigApplicationContext ctx = new AnnotationConfigApplicationContext(Example.class);
System.out.println(ctx.getBean(HttpClientImpl.class).getClass());
}
}
interface HttpClient {
void doGetAsync();
}
#Component
class HttpClientImpl implements HttpClient {
#Async
public void doGetAsync() {
System.out.println(Thread.currentThread());
}
}
Here, Spring attempts to find a bean of type HttpClientImpl which we expect to find because the type is clearly annotated with #Component. However, instead, we get an exception
Exception in thread "main" org.springframework.beans.factory.NoSuchBeanDefinitionException:
No qualifying bean of type [com.example.HttpClientImpl] is defined
Spring wrapped the HttpClientImpl bean and exposed it through a Proxy object that only implements HttpClient. So you could retrieve it with
ctx.getBean(HttpClient.class) // returns a dynamic class: com.example.$Proxy33
// or
#Autowired private HttpClient httpClient;
It's always recommended to program to interfaces. When you can't, you can tell Spring to use CGLIB proxies. For example, with #EnableAsync, you can set proxyTargetClass to true. Similar annotations (EnableTransactionManagement, etc.) have similar attributes. XML will also have equivalent configuration options.
ApplicationContext Hierarchies - Spring MVC
Spring lets you build ApplicationContext instances with other ApplicationContext instances as parents, using ConfigurableApplicationContext#setParent(ApplicationContext). A child context will have access to beans in the parent context, but the opposite is not true. This post goes into detail about when this is useful, particularly in Spring MVC.
In a typical Spring MVC application, you define two contexts: one for the entire application (the root) and one specifically for the DispatcherServlet (routing, handler methods, controllers). You can get more details here:
Difference between applicationContext.xml and spring-servlet.xml in Spring Framework
It's also very well explained in the official documentation, here.
A common error in Spring MVC configurations is to declare the WebMVC configuration in the root context with #EnableWebMvc annotated #Configuration classes or <mvc:annotation-driven /> in XML, but the #Controller beans in the servlet context. Since the root context cannot reach into the servlet context to find any beans, no handlers are registered and all requests fail with 404s. You won't see a NoSuchBeanDefinitionException, but the effect is the same.
Make sure your beans are registered in the appropriate context, ie. where they can be found by the beans registered for WebMVC (HandlerMapping, HandlerAdapter, ViewResolver, ExceptionResolver, etc.). The best solution is to properly isolate beans. The DispatcherServlet is responsible for routing and handling requests so all related beans should go into its context. The ContextLoaderListener, which loads the root context, should initialize any beans the rest of your application needs: services, repositories, etc.
Arrays, collections, and maps
Beans of some known types are handled in special ways by Spring. For example, if you tried to inject an array of MovieCatalog into a field
#Autowired
private MovieCatalog[] movieCatalogs;
Spring will find all beans of type MovieCatalog, wrap them in an array, and inject that array. This is described in the Spring documentation discussing #Autowired. Similar behavior applies to Set, List, and Collection injection targets.
For a Map injection target, Spring will also behave this way if the key type is String. For example, if you have
#Autowired
private Map<String, MovieCatalog> movies;
Spring will find all beans of type MovieCatalog and add them as values to a Map, where the corresponding key will be their bean name.
As described previously, if no beans of the requested type are available, Spring will throw a NoSuchBeanDefinitionException. Sometimes, however, you just want to declare a bean of these collection types like
#Bean
public List<Foo> fooList() {
return Arrays.asList(new Foo());
}
and inject them
#Autowired
private List<Foo> foos;
In this example, Spring would fail with a NoSuchBeanDefinitionException because there are no Foo beans in your context. But you didn't want a Foo bean, you wanted a List<Foo> bean. Before Spring 4.3, you'd have to use #Resource
For beans that are themselves defined as a collection/map or array
type, #Resource is a fine solution, referring to the specific
collection or array bean by unique name. That said, as of 4.3,
collection/map and array types can be matched through Spring’s
#Autowired type matching algorithm as well, as long as the element
type information is preserved in #Bean return type signatures or
collection inheritance hierarchies. In this case, qualifier values can
be used to select among same-typed collections, as outlined in the
previous paragraph.
This works for constructor, setter, and field injection.
#Resource
private List<Foo> foos;
// or since 4.3
public Example(#Autowired List<Foo> foos) {}
However, it will fail for #Bean methods, ie.
#Bean
public Bar other(List<Foo> foos) {
new Bar(foos);
}
Here, Spring ignores any #Resource or #Autowired annotating the method, because it's a #Bean method, and therefore can't apply the behavior described in the documentation. However, you can use Spring Expression Language (SpEL) to refer to beans by their name. In the example above, you could use
#Bean
public Bar other(#Value("#{fooList}") List<Foo> foos) {
new Bar(foos);
}
to refer to the bean named fooList and inject that.
lets say I have code like this:
#Repository
public class Foo{
}
#Service
public class Boo{
#Autowired
private Foo foo;
}
so now what here are we calling bean? Bean is the object of Foo type of refrence "foo" BUT are Boo class annotated as Service and Foo as Repository ALSO beans? Ihve been using spring for a while now but this basic question makes me feel bad for not knowing...
In the context of Spring, A bean is a spring managed object. Here spring managed means an object created, initialised, managed, destroyed by Spring IoC container.
Whenever we mark a class with #Component, Spring IOC container will create object for your class and manage it, Whenever we can simply get it from ApplicationContext, or access it using #Autowired/#Resource/#Inject annotations
We can also use #Controller, #Repository, #Service, #ControllerAdvice, #Configuration,#Aspect in place of #Component to tell more specifically that our class is a service or a repository or an aspect etc.
We can also use #Bean annotation to create a bean from method return value
#Configuration
public class SolrConfig {
#Value("${spring.data.solr.host}") String solrUrl;
#Bean
public SolrServer solrServer() {
return new HttpSolrServer(solrUrl);
}
#Bean(name = "solrTemplate")
public SolrTemplate solrTemplate() {
return new SolrTemplate(new HttpSolrServer(solrUrl), RULE_ENGINE_CORE);
}
}
All of your application components (#Component, #Service, #Repository, #Controller etc.) will be automatically registered as Spring Beans
http://docs.spring.io/autorepo/docs/spring-boot/current/reference/html/using-boot-spring-beans-and-dependency-injection.html
Defining Beans can be thought of as replacing the keyword new.
Further information can be found here which might be helpful for understanding Beans in Spring.
#ComponentScan creates beans using both #Configuration and #Component. Both these annotations work fine when swapped. What is the difference then?
#Configuration Indicates that a class declares one or more #Bean
methods and may be processed by the Spring container to generate bean
definitions and service requests for those beans at runtime
#Component Indicates that an annotated class is a "component". Such
classes are considered as candidates for auto-detection when using
annotation-based configuration and classpath scanning.
#Configuration is meta-annotated with #Component, therefore
#Configuration classes are candidates for component scanning
You can see more here:
http://docs.spring.io/spring-framework/docs/4.0.4.RELEASE/javadoc-api/org/springframework/context/annotation/Configuration.html
A #Configuration is also a #Component, but a #Component cannot act like a #Configuration.
Actually answer is not complete, is it true that:
#Component Indicates that an annotated class is a "component". Such
classes are considered as candidates for auto-detection when using
annotation-based configuration and classpath scanning.
But you do can create i.e MyConfiguration.java class then stereotype with #Component and add #Beans declaration to it. In this way it will looks as a configuration, main difference is that when annotated class with #Configuration #Bean annotated methods are proxy using CGLIB which made in code calls after the first one to return bean from context instead of execute method again and create another instance as happens when using #Component with #Bean
There is a very subtle difference between them. Let me provide a very quick outlook to this.
Consider the below scenario:
#Configuration
public class MyConfig {
#Bean
public ServiceA aService(){
return new ServiceA();
}
#Bean
public ServiceB bService(){
return new ServiceB(aService());
}
}
Note that ServiceB bean has a dependecy on ServiceA and this is not autowired. Instead, the way it's written implies that a new instance is created, which is not actually created by Spring. You, the programmer, did it with the new keyword instead.
So, if we do use #Configuration, then it uses CGLIB proxying, and in this situation it creates a singleton bean managed by the Spring context. If you invoke it multiple times, it returns the same bean that was created by Spring - sort of autowiring effect.
Whereas if you use #Component, it won't do this proxying and will simply return a new instance every time the method is invoked, instead of providing the Spring managed instance. (Remember that a Spring bean is something that is managed by the Spring container, and, as a developer, it's your job is to pull them in, e.g. with #Autowired.
The same #Component effect can be achieved with #Configuration(proxyEnabled= false) (This is also referred to as bean light mode processing). So, in light mode, you would end up doing something like this:
#Configuration(proxyEnabled = false) // Lite mode, same effect as #Component
public class MyConfig {
#Bean
public ServiceA aService() {
return new ServiceA();
}
#Autowired
#Bean
public ServiceB bService(ServiceA aServiceBean){
return new ServiceB(aServiceBean);
}
}
Refer here for a more elaborate explanation
Hope that helps! Happy Coding!
#Configuration - It is like beans.xml but Java-based bean configuration. It means class annotated with this annotation is the place where beans are configured and will be a candidate for auto-detection. In this class, methods are annotated with #Bean which return an object of the class.
Example:
#Configuration
public class ConfigClass {
#Bean
public UserClass getObject() {
return new UserClass();
}
}
#Component - You cannot autowire (#Autowired) any class if it is not marked with #Component. It means when you want to autowire any class using annotation that class should be annotated with #Component.
Example:
#Component
public class A { .... }
public class B {
#Autowired
A a;
.....
.....
}
Spring Document for reference:
https://docs.spring.io/spring-framework/docs/current/javadoc-api/org/springframework/context/annotation/Configuration.html
#Component is imported by default with #Configuration. controllers, service, and repostory are children components (along with Configuration). They are also candidate for auto-detection.
I am extending on #reus's answer.
#Configuration Indicates that a class declares one or more #Bean methods and may be processed by the Spring container to generate bean definitions and service requests for those beans at runtime.
If you look at the #Configuration class, you will see that it is meta-annotated with #Component.
#Target(value=TYPE)
#Retention(value=RUNTIME)
#Documented
#Component
public #interface Configuration
#Bean is enables us to define the dependency in any way we like, this is why the #Bean annotation goes above a methods and we manually create a bean object and return it from that method. #Component enables us to define a dependency quickly, that is why #Component goes above classes. We only inject it wherever we need.
Collectively these 3 points says that- to quickly define a bean, we can annotate the class with #Component. To define a bean as we like (support custom requirements), we can write the bean definition using #Bean inside a #Configuration annotated class.
Apart from the differences highlighted by reos.
The reason why #Configuration cannot be replaced by #Component is as below:
The difference is in how the inter bean dependency is handled.
Refer the link for a detailed explanation with example:
Difference between Configuration and Component
Assume there are two implementations of a single interface, and these beans are declared as beans in the spring configuration xml. Now, I would need only one implementation of the interface based on the system property. And, I don't wanna create the second implementation of the bean. How can I do this? I looked at this blog but then below snippet of the code from this blog uses "new" operate to create the beans. In my case the beans are declared in the spring configuration file.
http://www.intertech.com/Blog/spring-4-conditional-bean-configuration/
#CONFIGURATION
PUBLIC CLASS MYCONFIGURATION {
#BEAN(NAME="EMAILERSERVICE")
#CONDITIONAL(WINDOWSCONDITION.CLASS)
PUBLIC EMAILSERVICE WINDOWSEMAILERSERVICE(){
RETURN NEW WINDOWSEMAILSERVICE();
}
#BEAN(NAME="EMAILERSERVICE")
#CONDITIONAL(LINUXCONDITION.CLASS)
PUBLIC EMAILSERVICE LINUXEMAILERSERVICE(){
RETURN NEW LINUXEMAILSERVICE();
}
In a Spring MVC proyect I'm using Spring Data in the persistence layer so I have a bunch of repositories to access the data. I also have a layer for services so I have things like UserService or AuthorityService that use that repositories.
The problem is that I've been asked to create an interface to be able to change the implementation of the persistence layer (using DAOs for example) without have to touch a single line in the services. How can I specify in an Autowired of that interface what implementation to use? I´m using Java based config and I don't see how to inject it.
I also have a problem with the name of these new interfaces. Normally I would use a name like UserService but Spring use Service for the service layer so, What name is suitable for this type of interface?
You could mark the new implementation of the DAO as #Primary. Indicates that a bean should be given preference when multiple candidates are qualified to autowire a single-valued dependency. If exactly one 'primary' bean exists among the candidates, it will be the autowired value.
#Component
public class FooService {
private FooRepository fooRepository;
#Autowired
public FooService(FooRepository fooRepository) {
this.fooRepository = fooRepository;
}
}
#Component
public class JdbcFooRepository {
public JdbcFooService(DataSource dataSource) {
// ...
}
}
#Primary
#Component
public class HibernateFooRepository {
public HibernateFooService(SessionFactory sessionFactory) {
// ...
}
}
Because HibernateFooRepository is marked with #Primary, it will be injected preferentially over the jdbc-based variant assuming both are present as beans within the same Spring application context, which is often the case when component-scanning is applied liberally.
This annotation is semantically equivalent to the element's primary attribute in Spring XML.
I didn't completely follow your second question.