I have been trying to wrap my head around IoC and dependency injection. I think I am starting to understand the benefit of dependency injection in large apps. If I'm correct in understanding, dependency injection is beneficial when you have consistent interfaces, as you'll be able to only change an implementation without having to change your whole app.
What I don't understand is where XML comes into all of this (And I know there are other IoC methods like annotations now, but unless the answer is "XML sucks and you should use annotations", let's not go there yet).
What is the difference between managing this:
public class MyApp {
public static void main(String[] args) {
ItemFinder theQuarterFinder = new QuarterItemFinder();
FortuneService theFortune = new HappyFortuneService(theQuarterFinder);
Coach theCoach = new WrestlingCoach(theFortune);
System.out.println(theCoach.getDailyWorkout());
System.out.println(theCoach.getDailyFortune());
System.out.println(theCoach.getItem());
}
}
And managing this with an xml:
public class HelloSpringApp {
public static void main(String[] args) {
//Load spring configuration file
ClassPathXmlApplicationContext context =
new ClassPathXmlApplicationContext("applicationContext.xml");
//Retrieve bean from spring container
Coach theCoach = context.getBean("myCoach", Coach.class);
System.out.println(theCoach.getDailyWorkout());
System.out.println(theCoach.getDailyFortune());
System.out.println(theCoach.getItem());
context.close();
}
}
<bean id="myItemFinder" class="com.luv2code.springdemo.QuarterItemFinder">
</bean>
<bean id="myFortune"
class="com.luv2code.springdemo.HappyFortuneService">
<constructor-arg ref="myItemFinder" />
</bean>
<bean id="myCoach" class="com.luv2code.springdemo.WrestlingCoach">
<constructor-arg ref="myFortune" />
</bean>
If I were to change the itemFinder implementation, I would be changing exactly the same amount of code, only doing it in an XML file vs the main app function. Is that really preferable? Trying to track down refs in XML seems worse to deal with than just changing one of the "new" calls in MyApp.
If I'm understanding correctly, both examples do dependency injection. One just moves the logic out of app and into an XML file. and creates Spring Beans. But we are kinda making beans even without an XML context, correct?
Here's an excerpt from section 12.2.1 of Dependency Injection Principles, Practices, and Patterns which explains the advantages and disadvantages of using (XML) configuration files.
12.2.1 Configuring containers with configuration files
When DI Containers first appeared back in the early 2000s, they all used XML as a configuration mechanism — most things did back then. Experience with XML as a configuration mechanism later revealed that this is rarely the best option.
XML tends to be verbose and brittle. When you configure a DI Container in XML, you identify various classes and interfaces, but you have no compiler support to warn you if you misspell something. Even if the class names are correct, there’s no guarantee that the required assembly is going to be in the application’s probing path.
To add insult to injury, the expressiveness of XML is limited compared to that of plain code. This sometimes makes it hard or impossible to express certain configurations in a configuration file that are otherwise trivial to express in code [...]
The advantage of configuration files, on the other hand, is that you can change the behavior of the application without recompilation. This is valuable if you develop software that ships to thousands of customers, because it gives them a way to customize the application. But if you write an internal application or a website where you control the deployment environment, it’s often easier to recompile and redeploy the application when you need to change the behavior.
To summarize: (XML) configuration files are (only) valuable in late-binding scenarios, where the types are unknown at compile-time, which most likely happens when allowing an application to be extended using plug-ins.
Is there a way to easily use Spring Injection to grab one or more *.xml data files from a folder location (either in a deployed *.war or on a server folder) and inject that data from the *.xml files into a Java class (e.g. in a web service)? I have been asked by another programmer if I can do this.
I've had a look at a few links on stackoverflow, but so far the easiest way I've found is to put the *.xml files into a particular folder location (e.g. WEB-INF/classes) and use something like this to retrieve them:
Thread.currentThread().getContextClassLoader.getResourceAsStream("/WEB-INF/classes/data.xml")
The above method is easy; however, it is obviously not Spring Injection. Is there a way to do this using Spring Injection instead? I would have thought that since configuration files can be loaded this way, that xml data could also be loaded similarly.
Thanks.
Spring provides a class called Resource which you can use to inject resource files into a spring bean. So you can do this:
public class Consumer {
public void setResource(Resource resource) {
DataInputStream resourceStream = new DataInputStream(resource.getInputStream());
// ... use the stream as usual
}
...
}
Then:
<bean class="Consumer">
<property name="resource" value="classpath:path/to/file.xml"/>
</bean>
or,
<bean class="Consumer">
<property name="resource" value="file:path/to/file.xml"/>
</bean>
You can also directly use the #Value annotation:
public class Consumer {
#Value("classpath:path/to/file.xml")
private Resource resource;
...
}
I am a developer working on a Java web application that is built on the Spring framework.
This application will be deployed to several different customers.
There is a class which contains some business logic that is different for each client.
From a Spring framework point of view, it is enough to simply wire in the appropriate class (as a bean) for each client.
However, the developers are not responsible for deployment of the application. The Operations team is responsible, and having them open up the WAR file and modify the spring configuration XML for each client deployment is probably too much to ask from them. Properties files are ok, but modifying internal files - probably not.
Has anyone else come up with a strategy for dealing with this?
Edit:
To give an example of what I'm talking about:
public interface IEngine {
void makeNoise();
}
public class Car {
public void setEngine(IEngine engine) {
this.engine = engine;
}
}
Customer A's business logic:
HeavyDutyEngine implements IEngine {
public void makeNoise() {
System.out.println("VROOOM!");
}
}
Customer B's business logic:
LightWeightEngine implements IEngine {
public void makeNoise() {
System.out.println("putputput");
}
}
In the Spring configuration XML:
For client A, it might look like this:
<bean id="hdEngine" class="HeavyDutyEngine" />
<bean id="lwEngine" class="LightWeightEngine" />
<bean id="car" class="Car">
<property name="engine" ref="hdDngine">
</bean>
For client B, it might look like this:
<bean id="hdEngine" class="HeavyDutyEngine" />
<bean id="lwEngine" class="LightWeightEngine" />
<bean id="car" class="Car">
<property name="engine" ref="lwEngine">
</bean>
To configure Spring for different environments, you can use the concept called spring "profiles". (introduced in Spring 3.1)
You have different ways so enable/disable this properties. For example java properties parameter.
But because you are using a WAR, and therefore some Servlet container, I would recommend to put this configuration in the Servlet container.
In a tomcat for example, you can put this line in the context.xml (global or application specific) to enable a profile
<Parameter name="spring.profiles.active" value="myProfile"/>
You can move some configuration to DB, if you are using one.
And you can use something like this
ref="{ref-name}"
where ref-name can be resolved using properties file(default) by configuring PropertyPlaceholderConfigurer.
Or you can write your own wrapper over PropertyPlaceholderConfigurer which will take the values from DB table which is external to you deployable WAR file.
In one of mine project, we used this method to resolve custom dependencies. The wrapper which looks up the DB used to take first priority and if the DB doesn't have the key/value pair, then properties file (bundled in the WAR) was used to resolve dependencies.
This will also allow you to change some value externally from DB, however with IBM Websphere we need to recycle the server for changes to take place.
I have a static method I have no control over: MyUtils.getServiceURL() which returns a URL. I need to extract just the port from this URL to set a property on a bean:
<bean id="myBean>
<property name="defaultPort" value="?????"/>
</bean>
I can get the string value of the URL as a bean like this:
<bean id="serviceURL" class="MyUtils" factory-method="getServiceURL" />
I'm probably just going to write a method to parse the URL in myBean and extract the port. I was wondering if there was a way to avoid having to do this, and configure this directly in the spring context?
No need for custom classes, or parsers. Remember, a bean is just a class with get/is/set methods. A java.net.URL class qualifies as such.
Solution: Use a java.net.URL as a Spring-configurable bean and call it's getPort method.
Remember, in Spring, you can wire anything as a bean very easily so long as it sports methods using the get/is/set Bean convention. The java.net.URL class does such a thing.
** warning ** I'm just typing this out of my a$$, you'll have to check for any syntax shenanigans that I might be introducing in this Spring-config pseudo-code. One thing that is certain is that the concept will work with Spring 2.5 and that it relies on the util schema.
If you are using an older version of Spring, you'll have to use a PropertyPathFactoryBean. If you are using Spring 3.0, you'll want to use Spring expression language (EL), but I can't comment on the later since I'm not familiar with Spring 3.0.
In java:
int port = (new URL(MyUtils.getServiceURL()).getPort()
In Spring:
<bean id="serviceURL" class="MyUtils" factory-method="getServiceURL" />
<bean id="myURL" class="java.net.URL">
<constructor-arg>
<ref-bean="serviceURL"/>
</constructor-arg>
</bean>
<util:property-path id="defaultPort" path="myURL.port"/>
<bean id="myBean>
<property name="defaultPort" ref="defaultPort"/>
</bean>
There might be a way to consolidate all these four expressions into three or less, don't know if that will improve readability, though. The concept remains the same, though. Treat a java.net.URL instance as a bean, set its URL via its constructor, and get a hold (via Spring) of its getPort() method.
** edit **:
If you are using Spring 2.5 you can create an inline groovy bean that does all that for you. I've seen people doing that as integration glue to get rid of multitude of temp Spring bean place holders. That's the best choice IMO when you need to define a class (when it's simpler than just using Spring tags) and when you know such a class won't be used outside of Spring wiring of things.
Not I can think of. Either way, you probably have to write something to parse the port out. Since you have no control of that static method, perhaps you may want to utilize the adapter pattern to hide that static method. Doing so also makes your code easier to test because you can easily mock that external API out. For example:-
public interface URLServiceAdapter {
int getServiceURLPort();
}
public class URLServiceAdapterImpl implements URLServiceAdapter {
public int getServiceURLPort() {
return MyUtils.getServiceURL().getPort();
}
}
Your beans will then rely on your adapter class instead of MyUtils.getServiceURL() directly:-
<bean id="urlServiceAdapter" class="project.URLServiceAdapterImpl" />
That way, you can do something like this:-
#Autowired
private UrlServiceAdapter urlServiceAdapter;
...
bean.setDefaultPort(urlServiceAdapter.getServiceURLPort());
SpEL is your friend.
<bean id="myBean>
<property name="defaultPort"
value='#{ T(com.foo.MyUtils).getServiceURL().split(":")[2].split("/")[0] }'/>
</bean>
Enjoy,
Consider the following scenario. I have a Spring application context with a bean whose properties should be configurable, think DataSource or MailSender. The mutable application configuration is managed by a separate bean, let's call it configuration.
An administrator can now change the configuration values, like email address or database URL, and I would like to re-initialize the configured bean at runtime.
Assume that I can't just simply modify the property of the configurable bean above (e.g. created by FactoryBean or constructor injection) but have to recreate the bean itself.
Any thoughts on how to achieve this? I'd be glad to receive advice on how to organize the whole configuration thing as well. Nothing is fixed. :-)
EDIT
To clarify things a bit: I am not asking how to update the configuration or how to inject static configuration values. I'll try an example:
<beans>
<util:map id="configuration">
<!-- initial configuration -->
</util:map>
<bean id="constructorInjectedBean" class="Foo">
<constructor-arg value="#{configuration['foobar']}" />
</bean>
<bean id="configurationService" class="ConfigurationService">
<property name="configuration" ref="configuration" />
</bean>
</beans>
So there's a bean constructorInjectedBean that uses constructor injection. Imagine the construction of the bean is very expensive so using a prototype scope or a factory proxy is not an option, think DataSource.
What I want to do is that every time the configuration is being updated (via configurationService the bean constructorInjectedBean is being recreated and re-injected into the application context and dependent beans.
We can safely assume that constructorInjectedBean is using an interface so proxy magic is indeed an option.
I hope to have made the question a little bit clearer.
Here is how I have done it in the past: running services which depend on configuration which can be changed on the fly implement a lifecycle interface: IRefreshable:
public interface IRefreshable {
// Refresh the service having it apply its new values.
public void refresh(String filter);
// The service must decide if it wants a cache refresh based on the refresh message filter.
public boolean requiresRefresh(String filter);
}
Controllers (or services) which can modify a piece of configuration broadcast to a JMS topic that the configuration has changed (supplying the name of the configuration object). A message driven bean then invokes the IRefreshable interface contract on all beans which implement IRefreshable.
The nice thing with spring is that you can automatically detect any service in your application context that needs to be refreshed, removing the need to explicitly configure them:
public class MyCacheSynchService implements InitializingBean, ApplicationContextAware {
public void afterPropertiesSet() throws Exception {
Map<String, ?> refreshableServices = m_appCtx.getBeansOfType(IRefreshable.class);
for (Map.Entry<String, ?> entry : refreshableServices.entrySet() ) {
Object beanRef = entry.getValue();
if (beanRef instanceof IRefreshable) {
m_refreshableServices.add((IRefreshable)beanRef);
}
}
}
}
This approach works particularly well in a clustered application where one of many app servers might change the configuration, which all then need to be aware of. If you want to use JMX as the mechanism for triggering the changes, your JMX bean can then broadcast to the JMS topic when any of its attributes are changed.
I can think of a 'holder bean' approach (essentially a decorator), where the holder bean delegates to holdee, and it's the holder bean which is injected as a dependency into other beans. Nobody else has a reference to holdee but the holder. Now, when the holder bean's config is changed, it recreates the holdee with this new config and starts delegating to it.
You should have a look at JMX. Spring also provides support for this.
Spring 2.0.x
Spring 2.5.x
Spring 3.0.x
Further updated answer to cover scripted bean
Another approach supported by spring 2.5.x+ is that of the scripted bean. You can use a variety of languages for your script - BeanShell is probably the most intuitive given that it has the same syntax as Java, but it does require some external dependencies. However, the examples are in Groovy.
Section 24.3.1.2 of the Spring Documentation covers how to configure this, but here are some salient excerpts illustrating the approach which I've edited to make them more applicable to your situation:
<beans>
<!-- This bean is now 'refreshable' due to the presence of the 'refresh-check-delay' attribute -->
<lang:groovy id="messenger"
refresh-check-delay="5000" <!-- switches refreshing on with 5 seconds between checks -->
script-source="classpath:Messenger.groovy">
<lang:property name="message" value="defaultMessage" />
</lang:groovy>
<bean id="service" class="org.example.DefaultService">
<property name="messenger" ref="messenger" />
</bean>
</beans>
With the Groovy script looking like this:
package org.example
class GroovyMessenger implements Messenger {
private String message = "anotherProperty";
public String getMessage() {
return message;
}
public void setMessage(String message) {
this.message = message
}
}
As the system administrator wants to make changes then they (or you) can edit the contents of the script appropriately. The script is not part of the deployed application and can reference a known file location (or one that is configured through a standard PropertyPlaceholderConfigurer during startup).
Although the example uses a Groovy class, you could have the class execute code that reads a simple properties file. In that manner, you never edit the script directly, just touch it to change the timestamp. That action then triggers the reload, which in turn triggers the refresh of properties from the (updated) properties file, which finally updates the values within the Spring context and off you go.
The documentation does point out that this technique doesn't work for constructor-injection, but maybe you can work around that.
Updated answer to cover dynamic property changes
Quoting from this article, which provides full source code, one approach is:
* a factory bean that detects file system changes
* an observer pattern for Properties, so that file system changes can be propagated
* a property placeholder configurer that remembers where which placeholders were used, and updates singleton beans’ properties
* a timer that triggers the regular check for changed files
The observer pattern is implemented by
the interfaces and classes
ReloadableProperties,
ReloadablePropertiesListener,
PropertiesReloadedEvent, and
ReloadablePropertiesBase. None of them
are especially exciting, just normal
listener handling. The class
DelegatingProperties serves to
transparently exchange the current
properties when properties are
updated. We only update the whole
property map at once, so that the
application can avoid inconsistent
intermediate states (more on this
later).
Now the
ReloadablePropertiesFactoryBean can be
written to create a
ReloadableProperties instance (instead
of a Properties instance, as the
PropertiesFactoryBean does). When
prompted to do so, the RPFB checks
file modification times, and if
necessary, updates its
ReloadableProperties. This triggers
the observer pattern machinery.
In our case, the only listener is the
ReloadingPropertyPlaceholderConfigurer.
It behaves just like a standard spring
PropertyPlaceholderConfigurer, except
that it tracks all usages of
placeholders. Now when properties are
reloaded, all usages of each modified
property are found, and the properties
of those singleton beans are assigned
again.
Original answer below covering static property changes:
Sounds like you just want to inject external properties into your Spring context. The PropertyPlaceholderConfigurer is designed for this purpose:
<!-- Property configuration (if required) -->
<bean id="serverProperties" class="org.springframework.beans.factory.config.PropertyPlaceholderConfigurer">
<property name="locations">
<list>
<!-- Identical properties in later files overwrite earlier ones in this list -->
<value>file:/some/admin/location/application.properties</value>
</list>
</property>
</bean>
you then reference the external properties with Ant syntax placeholders (that can be nested if you want from Spring 2.5.5 onwards)
<bean id="example" class="org.example.DataSource">
<property name="password" value="${password}"/>
</bean>
You then ensure that the application.properties file is only accessible to the admin user and the user running the application.
Example application.properties:
password=Aardvark
Or you could use the approach from this similar question and hence also my solution:
The approach is to have beans that are configured via property files and the solution is to either
refresh the entire applicationContext (automatically using a scheduled task or manually using JMX) when properties have changed or
use a dedicated property provider object to access all properties. This property provider will keep checking the properties files for modification. For beans where prototype-based property lookup is impossible, register a custom event that your property provider will fire when it finds an updated property file. Your beans with complicated lifecycles will need to listen for that event and refresh themselves.
You can create a custom scope called "reconfigurable" into the ApplicationContext. It creates and caches instances of all beans in this scope. On a configuration change it clears the cache and re-creates the beans on first access with the new configuration. For this to work you need to wrap all instances of reconfigurable beans into an AOP scoped proxy, and access the configuration values with Spring-EL: put a map called config into the ApplicationContext and access the configuration like #{ config['key'] }.
This is not something I tried, I am trying to provide pointers.
Assuming your application context is a subclass of AbstractRefreshableApplicationContext(example XmlWebApplicationContext, ClassPathXmlApplicationContext). AbstractRefreshableApplicationContext.getBeanFactory() will give you instance of ConfigurableListableBeanFactory. Check if it is instance of BeanDefinitionRegistry. If so you can call 'registerBeanDefinition' method. This approach will be tightly coupled with Spring implementation,
Check the code of AbstractRefreshableApplicationContext and DefaultListableBeanFactory(this is the implementation you get when you call 'AbstractRefreshableApplicationContext getBeanFactory()')
Option 1 :
Inject the configurable bean into the DataSource or MailSender. Always get the configurable values from the configuration bean from within these beans.
Inside the configurable bean run a thread to read the externally configurable properties (file etc..) periodically. This way the configurable bean will refresh itself after the admin had changed the properties and so the DataSource will get the updated values automatically.
You need not actually implement the "thread" - read : http://commons.apache.org/configuration/userguide/howto_filebased.html#Automatic_Reloading
Option 2 (bad, i think, but maybe not - depends on use case) :
Always create new beans for beans of type DataSource / MailSender - using prototype scope. In the init of the bean, read the properties afresh.
Option 3 :
I think, #mR_fr0g suggestion on using JMX might not be a bad idea. What you could do is :
expose your configuration bean as a MBean (read http://static.springsource.org/spring/docs/2.5.x/reference/jmx.html)
Ask your admin to change the configuration properties on the MBean (or provide an interface in the bean to trigger property updates from their source)
This MBean (a new piece of java code that you will need to write), MUST keep references of Beans (the ones that you want to change / inject the changed properties into). This should be simple (via setter injection or runtime fetch of bean names / classes)
When the property on the MBean is changed (or triggered), it must call the appropriate setters on the respective beans. That way, your legacy code does not change, you can still manage runtime property changes.
HTH!
You may want to have a look at the Spring Inspector a plug-gable component that provides programmatic access to any Spring based application at run-time. You can use Javascript to change configurations or manage the application behaviour at run-time.
Here is the nice idea of writing your own PlaceholderConfigurer that tracks the usage of properties and changes them whenever a configuration change occurs. This has two disadvantages, though:
It does not work with constructor injection of property values.
You can get race conditions if the reconfigured bean receives a
changed configuration while it is processing some stuff.
My solution was to copy the original object. Fist i created an interface
/**
* Allows updating data to some object.
* Its an alternative to {#link Cloneable} when you cannot
* replace the original pointer. Ex.: Beans
* #param <T> Type of Object
*/
public interface Updateable<T>
{
/**
* Import data from another object
* #param originalObject Object with the original data
*/
public void copyObject(T originalObject);
}
For easing the implementation of the function fist create a constructor with all fields, so the IDE could help me a bit. Then you can make a copy constructor that uses the same function Updateable#copyObject(T originalObject). You can also profit of the code of the constructor created by the IDE to create the function to implement:
public class SettingsDTO implements Cloneable, Updateable<SettingsDTO>
{
private static final Logger LOG = LoggerFactory.getLogger(SettingsDTO.class);
#Size(min = 3, max = 30)
private String id;
#Size(min = 3, max = 30)
#NotNull
private String name;
#Size(min = 3, max = 100)
#NotNull
private String description;
#Max(100)
#Min(5)
#NotNull
private Integer pageSize;
#NotNull
private String dateFormat;
public SettingsDTO()
{
}
public SettingsDTO(String id, String name, String description, Integer pageSize, String dateFormat)
{
this.id = id;
this.name = name;
this.description = description;
this.pageSize = pageSize;
this.dateFormat = dateFormat;
}
public SettingsDTO(SettingsDTO original)
{
copyObject(original);
}
#Override
public void copyObject(SettingsDTO originalObject)
{
this.id = originalObject.id;
this.name = originalObject.name;
this.description = originalObject.description;
this.pageSize = originalObject.pageSize;
this.dateFormat = originalObject.dateFormat;
}
}
I used it in a Controller for updating the current settings for the app:
if (bindingResult.hasErrors())
{
model.addAttribute("settingsData", newSettingsData);
model.addAttribute(Templates.MSG_ERROR, "The entered data has errors");
}
else
{
synchronized (settingsData)
{
currentSettingData.copyObject(newSettingsData);
redirectAttributes.addFlashAttribute(Templates.MSG_SUCCESS, "The system configuration has been updated successfully");
return String.format("redirect:/%s", getDao().getPath());
}
}
So the currentSettingsData which has the configuration of the application gonna have the updated values, located in newSettingsData. These method allows updating any bean without high complexity.