Given a Spring configuration that exclusively contains eager (non-lazy) singleton beans, i.e. the defaults, is it possible to have Spring throw an exception in the case where any of those beans is not injected anywhere? I'm essentially looking for a way to detect dead code in the form of Spring beans.
My question is somewhat similar to these.
http://forum.spring.io/forum/spring-projects/container/116494-any-tools-or-method-to-identify-unused-spring-beans
Spring Instantiation and 'unused beans'
How to detect unused properties in Spring
However,
I'm not interested in manually inspecting a graph or parsing log data.
I don't have the added complexity of multiple context files, overriding beans, bean post-processing, or xml. It's a simple, straightforward, annotation-driven configuration.
I'm using Spring Boot 1.2.6 which is several years newer than those questions (maybe new functionality exists).
Spring will certainly throw an exception if a necessary bean is missing. Can it also throw an exception in the opposite scenario where a bean is found but unnecessary?
Spring will certainly throw an exception if a necessary bean is
missing. Can it also throw an exception in the opposite scenario where
a bean is found but unnecessary?
TL/DR:
Spring does not support this (and probably never will).
Long version:
Detecting if a bean is used can be really hard.
First, lets define when does spring throw the "missing bean" exception.
During the initialisation of the spring context, spring creates the beans in the order in which it will allow for all dependencies to be satisfied (if possible). If a bean is missing a dependency, spring will throw an exception (as you said).
So, the exception is thrown during the spring context initialisation process.
Now, you could say that we could monitor this process and look for a bean that was not used as a dependency in any other bean.
The problem is that not all bean dependencies are defined during the spring context initialisation process.
Let's look at the following example:
First, we have a simple interface, DataService
public interface DataService {
String getData();
}
Now we have 2 spring beans that implement this interface:
#Service("firstDataService")
public class FirstDataService implements DataService {
#Override
public String getData() {
return "FIRST DATA SERVICE";
}
}
#Service("secondDataService")
public class SecondDataService implements DataService {
#Override
public String getData() {
return "SECOND DATA SERVICE";
}
}
Now, imagine that there is no bean that depends on these two beans directly. When I say directly, I mean there is no bean that depends on these beans via constructor-based, setter-based or field-based dependency injection.
Because of that, spring will not inject these beans inside any other bean during the context initialisation process.
Now, consider the following bean:
#Service
public class DataCollector {
#Autowired
ApplicationContext applicationContext;
String getDataFromService(String beanName) {
DataService ds = (DataService) applicationContext.getBean(beanName);
return ds.getData();
}
}
If I call the getDataFromService method of the DataCollector bean with "firstDataService" value for the beanName parameter, the method will return "FIRST DATA SERVICE" as a result.
If I call the method with "secondDataService", I will return "SECOND DATA SERVICE" as a result.
Now, when spring looks at the definition of DataController during context initialisation, there is no way to determine on which beans DataCollector depends on.
It all depends on the application logic, and the value that we use for the beanName parameter when we call the getDataFromService method.
Because of that, spring is not capable of determining if there is bean that is never used (because the bean usage can be dynamic, like in the case above).
Related
Anyone can explain how Spring defines the bean creation mechanism when create a bean which depends on a list of other beans? It would be good to show the part of Spring specification on how it's defined.
Code like:
public interface Test {
}
#Service
public class TestImpl1 implements Test{
}
#Service
public class TestImpl2 implements Test{
}
public class TestContainer {
List<Test> testList;
TestContainer() {
testList = new ArrayList<>();
}
public void addTest(Test test) {
testList.add(test);
}
}
then
#Bean
public TestContainer testContainer(List<Test> testList) {
TestContainer testContainer = new TestContainer();
for (Test test : testList) {
testContainer.addTest(test);
}
return testContainer;
}
Question is really: when creating bean for TestContainer, how does Spring figure out what should be in List testList?
This looks like what you are looking for:
https://docs.spring.io/spring/docs/current/spring-framework-reference/core.html#beans-factory-class
You haven't specified what exactly do you want to know about bean creation but here is the minimum you should know. By default all beans are singleton scoped (only created once during the container life-cycle and for all the subsequent request the same instance is returned). All singleton scoped beans are created eagerly. If the singleton bean is dependent on some other beans (needs them for instantiation) then those other beans will be instantiated with it/right before it, doesn't matter whether they are singletons or not, marked as lazy or not.
This is part of the spring documentation I think on their website. Look for it. The whole spring documentation is worth to read, even if it is a long read (https://docs.spring.io/spring/docs/current/spring-framework-reference/core.html#spring-core)
But mostly, spring would first read all the XML, anotations and other source of config data and get a list of beans to init. Then it would get a list of dependencies between beans constructing kind of a tree. As the dependencies need to be initialized first, there an obvious orderining for initialization.
On top, you can set your own priorities for beans so that you can get some beans initialized first or last for example.
For your specific case, spring will inspect the code source as well as use the Java reflection API to figure out you want a collection of interface Test implementations. So spring would look for ALL the defined bean that match and return them that not more complex than that.
It's mostly using the concepts of graph data structure where the beans become the nodes of the graph and they are resolved using topological sort.
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.
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.
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.
If I have a Spring bean that contains another non Spring bean like so :
#Component
class SpringBean {
private MyBean mb = new MyBean();
}
I receive error : unable to wire bean MyBean
But if I declare the Bean within a method :
#Component
class SpringBean {
private void myMethod(){
MyBean mb = new MyBean();
}
}
Then the bean is initialized correctly when the bean is called.
So does Spring not allow non Spring beans to be created by Spring beans ? What is the reasoning behind this?
Only beans created by the Spring container will be managed by Spring and provide dependency injection and all the benefits (and problems) that Spring gives you.
So does Spring not allow non Spring beans to be created by Spring beans ?
Spring beans are POJOs after all, so they can be manually created like any class with a public constructor. This means, you can do:
public void foo() {
SpringBean springBeanNotManagedBySpring = new SpringBean();
//use springBeanNotManagedBySpring...
}
But, as stated at the beginning of my answer, springBeanNotManagedBySpring is not managed by Spring, so it won't have the benefits of a Spring managed bean.
Note that you're doing two different things in your example:
In the first code, you have a field in the class of type MyBean and Spring tries to inject it but looks like doesn't find a Spring bean to inject. You should post the configuration of your bean in order for us to do a better analysis on this.
In the second code, you're creating a local variable (completely different from a field in class) of type MyBean, where the instantiation is up to the programmer.