I have a parent bean and a child bean in the spring configuration class like attached. How can I inject the child bean dynamically into parent bean based on some condition (like feature toggle).
#Configuration
public class FooConfig {
#Bean
public void parentBean(#Qualifier("dependantBean") Object bean){
//use the correct bean at runtime
}
#Bean("dependantBean")
#FeatureToggle(feature = "feature.one", expectedToBeOn = true)
public Object test1(){
//some logic and returns a object
return new Object();
}
#Bean("dependantBean")
#FeatureToggle(feature = "feature.one",expectedToBeOn = false)
public Object test2(){
//some logic and returns a object which is different from test1 method
return new Object();
}
}
In your example it seems that the decision would be taken when the application starts.
In that case you can handle what bean gets injected with profiles (#Profile).
Side note: Use different names for the method names that create the same bean. The bean name could be the same in this case.
Another alternative could be to create a Map with the 2 dependant beans, and inject the map into the parent bean constructor.
Based on the logic you want you can use any of the two beans in the Map.
In this case use different names for the the two dependant beans.
In an Spring Application it is possible to retrieve all (?) Beans with applicationContext.getBeansOfType(Object.class). This is of course only possible, after all Beans have been created.
So, if I call this method in the constructor of a Bean, I have to be lucky, to be the last Bean to be created, to have access to all of them.
As far as I understand the life cycle of Spring Beans, there is a phase in which BeanDefinitions are created, before the Beans are initialized.
How is it possible to retrive all created BeanDefinitions in the constructor of a Bean?
Can I also retrive the types (as Class) of those BeanDefinitions? The type BeanDefinition seems to only provide the "current bean class name of this bean definition".
Or is the only way to get those types after all Beans have been constructed (e.g. #PostConstruct)?
Maybe this code could help
for (String name : applicationContext.getBeanFactory().getBeanDefinitionNames()) {
BeanDefinition beanDefinition = applicationContext.getBeanFactory().getBeanDefinition(name);
String className = beanDefinition.getBeanClassName();
Class<?> clazz = Class.forName(className);
}
The loop gets you all the BeanDefinitions and then you load the class for each and do what you want?
By the way this might not be a good way to use Spring but it will work.
You can create a last bean by putting it for example in an #Configuration class with a minimum initialization order, so that it is the last one with
#Order(Ordered.LOWEST_PRECEDENCE), that would be it:
#Configuration
#Order(Ordered.LOWEST_PRECEDENCE)
public class Last {
#Autowired
ApplicationContext applicationContext;
#Bean
public String lastBean() {
applicationContext.getBeanDefinitionNames(); //retrive all created BeanDefinitions in the constructor of a Bean
applicationContext.getBeansOfType(Object.class); //retrive the types (as Class)
return "lastBean";
}
}
I've read some articles on optional bean dependencies, it is usually suggested to use java Optional class, or spring ObjectProvider class.
Those do work, but my case is a little different. What if in my context there are multiple beans of the same type, which annotated with #Qualifier and I don't know if there is a bean I need among them. And I need the one with the specific name.
#ComponentScan(basePackages = "my.package")
public class MyClass {
private final MyOptionalBean myOptionalBean;
MyClass(ObjectProvider<MyOptionalBean> myOptionalBeanObjectProvider) {
this.myOptionalBean = myOptionalBeanObjectProvider.getIfAvailable(() -> null);
}
}
The example above works. But now imagine, that there are multiple MyOptionalBean beans registered in my context, those beans are named. How do I write the similar code as above, but tell spring to look by the name of specific instance?
You can use #Autowired(required = false) and #Qualifier in combination:
MyClass(#Autowired(required = false) #Qualifier("foo") MyOptionalBean myBean)
{
// myBean will be null if no bean with the qualifier exists
}
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.
In my #Configuration file, I have beans with a relationship similar to the following:
#Bean
public Cache<Foo,Bar> fooBarCache(Map<Foo,Future<Bar>> refreshTaskMap) {
return new AsyncUpdateCache(refreshTaskMap);
}
#Bean
public Map<Foo,Future<Bar>> refreshTaskMap() {
return new HashMap<Foo,Future<Bar>>();
}
However, the ApplicationContext fails to load because it complains about "No qualifying bean of type [com.example.Bar]". From what I can tell, Spring is trying to be cute with creating a map for me and assumes I intend to use the map to lookup beans, or something similar.
How do I prevent it from trying to do its collection-injection "magic" and just inject the bean as I've declared it? I've tried adding a #Qualifier annotation on the fooBarCache argument, but that didn't seem to help.
You can use another approach to bean dependency injection - invoking the bean factory method instead of taking it in as a parameter (Spring doc). Then your configuration would look like this:
#Bean
public Cache<Foo,Bar> fooBarCache() {
return new AsyncUpdateCache(refreshTaskMap()); // call the method
}
#Bean
public Map<Foo,Future<Bar>> refreshTaskMap() {
return new HashMap<Foo,Future<Bar>>();
}
Spring is smart enough to realize that you want to use the bean refreshTaskMap and not simply call the method and instead of creating a new and unmanaged map instance it will replace the call with a lookup of an existing refreshTaskMap bean. That is described further here.
If you intend to autowire refreshTaskMap in other beans (outside of this configuration class), the Springs semantics of #Autowire Map<String, V> is to autowire a map of all beans of the type V, where keys are the bean names (map key type must be String) (reference)
Even typed Maps can be autowired as long as the expected key type is String. The Map values will contain all beans of the expected type, and the keys will contain the corresponding bean names
In that case, you should use #Resource:
beans that are themselves defined as a collection or map type cannot be injected through #Autowired, because type matching is not properly applicable to them. Use #Resource for such beans, referring to the specific collection or map bean by unique name.