Display deployed version on your maven powered Spring application

March 6, 2018March 6, 2018 by Dmitri Slinkov | Leave a comment

Sometimes it is useful to have an endpoint that will return your application’s maven build properties.

To set that up, there are only a few steps that need to be followed.

First, define the resource directory. By default, Maven will look in src/main/resources. Regardless, it is a good idea to add the resources and set filtering to true.

<resources>
	<resource>
		<directory>src/main/resources</directory>
		<filtering>true</filtering>
	</resource>
</resources>

<directory>src/main/resources</directory>

</resource>

</resources>

You will need to include Maven resources plugin. Using @ helps with avoiding collisions with Spring placeholders.

<plugin>
	<groupId>org.apache.maven.plugins</groupId>
	<artifactId>maven-resources-plugin</artifactId>
	<version>3.0.2</version>
	<configuration>
		<delimiters>
			<delimiter>@</delimiter>
		</delimiters>
		<useDefaultDelimiters>false</useDefaultDelimiters>
	</configuration>
</plugin>

<groupId>org.apache.maven.plugins</groupId>

<artifactId>maven-resources-plugin</artifactId>

</delimiters>

<useDefaultDelimiters>false</useDefaultDelimiters>

</configuration>

</plugin>

Setting up WordPress on Amazon Lightsail

March 2, 2018March 4, 2018 by Dmitri Slinkov | 1 Comment

After an unsuccessful attempt to start blogging, I decided to give it another try. And while my site with my 4 blog entries was still up and running, I decided to find it a new home. I have been working with AWS for awhile now and thought that Amazon Lightsail would be the perfect location.

Getting up and running was simple. Figuring out the details took a bit more effort. I decided to compile what I learned into a single easily accessible page.

Setting up Amazon Lightsail Account

For $5 (as of this writing) you will get a basic instance with 512 MB of RAM, 1 vCPU and 20 GB SSD. Head over to https://aws.amazon.com/lightsail/ and get yourself an account. You will need a credit or a debit card to sign up but you can try it out risk free for 30 days. Once you have your account, head over to Lightsail home and create an instance. Choose your instance location, platform (Linux) and a bluepring (WordPress).

Check whether you have multiple ContextLoader* definitions in your web.xml!

June 19, 2015March 4, 2018 by Dmitri Slinkov | 1 Comment

If you are using Spring in your web project and just added Spring Security, you may come across this error message when trying to start the application.



SEVERE: Exception sending context initialized event to listener instance of class org.springframework.web.context.ContextLoaderListener
java.lang.IllegalStateException: Cannot initialize context because there is already a root application context present - check whether you have multiple ContextLoader* definitions in your web.xml!
	at org.springframework.web.context.ContextLoader.initWebApplicationContext(ContextLoader.java:264)
	at org.springframework.web.context.ContextLoaderListener.contextInitialized(ContextLoaderListener.java:106)
	at org.apache.catalina.core.StandardContext.listenerStart(StandardContext.java:4797)
	at org.apache.catalina.core.StandardContext.startInternal(StandardContext.java:5291)
	at org.apache.catalina.util.LifecycleBase.start(LifecycleBase.java:150)
	at org.apache.catalina.core.ContainerBase$StartChild.call(ContainerBase.java:1559)
	at org.apache.catalina.core.ContainerBase$StartChild.call(ContainerBase.java:1549)
	at java.util.concurrent.FutureTask$Sync.innerRun(FutureTask.java:334)
	at java.util.concurrent.FutureTask.run(FutureTask.java:166)
	at java.util.concurrent.ThreadPoolExecutor.runWorker(ThreadPoolExecutor.java:1145)
	at java.util.concurrent.ThreadPoolExecutor$Worker.run(ThreadPoolExecutor.java:615)
	at java.lang.Thread.run(Thread.java:724)

SEVERE: Exception sending context initialized event to listener instance of class org.springframework.web.context.ContextLoaderListener

java.lang.IllegalStateException: Cannot initialize context because there is already a root application context present - check whether you have multiple ContextLoader* definitions in your web.xml!

at org.springframework.web.context.ContextLoader.initWebApplicationContext(ContextLoader.java:264)

at org.springframework.web.context.ContextLoaderListener.contextInitialized(ContextLoaderListener.java:106)

at org.apache.catalina.core.StandardContext.listenerStart(StandardContext.java:4797)

at org.apache.catalina.core.StandardContext.startInternal(StandardContext.java:5291)

at org.apache.catalina.util.LifecycleBase.start(LifecycleBase.java:150)

at org.apache.catalina.core.ContainerBase$StartChild.call(ContainerBase.java:1559)

at org.apache.catalina.core.ContainerBase$StartChild.call(ContainerBase.java:1549)

at java.util.concurrent.FutureTask$Sync.innerRun(FutureTask.java:334)

at java.util.concurrent.FutureTask.run(FutureTask.java:166)

at java.util.concurrent.ThreadPoolExecutor.runWorker(ThreadPoolExecutor.java:1145)

at java.util.concurrent.ThreadPoolExecutor$Worker.run(ThreadPoolExecutor.java:615)

at java.lang.Thread.run(Thread.java:724)

7 Essential Eclipse Plugins

June 16, 2015June 16, 2015 by Dmitri Slinkov | 2 Comments

Every developer has a list of his or her useful Eclipse plugins. This is my list. This list is not meant to be complete and comprehensive. It is highly biased and opinionated list. There a lot of great plugins out there but this list falls under the “must have” for me personally.

1. Springsource Tool Suite (STS)

If you are using Spring in your development, STS is a must have. Ctrl+Shift+L feature (for quick search) alone is a great reason to use STS in your daily work.

2. StartExplorer Eclipse Plug-in

I found this plug-in to be great for quick access to a file outside of Eclipse. “This plug-in offers tight integration of system file manager (Windows Explorer, Gnome Nautilus, KDE Konqueror, Mac Finder, …) and shell (cmd.exe, Linux/Mac terminal) in Eclipse.”

Spring’s @Transactional Important Points

January 25, 2014March 2, 2018 by Dmitri Slinkov | 4 Comments

Spring has a great way to handle transactions using @Transactional annotation. However, there are a few gotchas that one needs to watch out for. Without a good understanding of how Spring treats transactions, you can run into some unexpected and undesirable behavior. For an in depth study of the topic refer to the Sping Spring Documentation on transaction management. I will try to summarize a few important points.

1. Spring provides support for transactions using AOP Proxies

It means that you are not calling a method directly but through a proxy. This is a very important point to remember when defining transaction boundaries. In an event where a service method calls another method in the same class with a different propagation setting (PROPAGATION_REQUIRES_NEW for example), that setting will be ignored since the call is made internally. If you are not aware of the AOP concept the behavior of those methods will most likely not match your expectations.

In order to let Spring manage transactions you have to add @EnableTransactionManagement to your Java config class or if using XML configuration add <tx:annotation-driven/> . (Don’t forget the namespace: xmlns:tx="http://www.springframework.org/schema/tx"

Converting JSON object to a generic class with Jackson

January 6, 2014 by Dmitri Slinkov | 1 Comment

Marshaling a Java Object to JSON is very straight forward. Here is a simple example that prints out the object’s JSON representation.

MyObject myObject = new MyObject();
ObjectMapper mapper = new ObjectMapper();
System.out.println(mapper.writeValueAsString(myObject));

MyObject myObject = new MyObject();

ObjectMapper mapper = new ObjectMapper();

System.out.println(mapper.writeValueAsString(myObject));

Converting a JSON String to a Java Object is similarly simple.

ObjectMapper mapper = new ObjectMapper();
MyObject myObject  = mapper.readValue(new StringReader(jsonString), MyObject.class);

1 2	ObjectMapper mapper = new ObjectMapper(); MyObject myObject = mapper.readValue(new StringReader(jsonString), MyObject.class);

Sometimes, however, it is necessary to unmarshal a JSON object into a generic class. To do that we need to use TypeReference.class.

Map<String, String> properties = new HashMap<>();
try {

    ObjectMapper mapper = new ObjectMapper();
    properties = mapper.readValue(jsonString, new TypeReference<Map<String, String>>() {
    });

} catch (IOException e) {   
    log.error(e.getMessage(), e);
}

Map<String, String> properties = new HashMap<>();

try {

ObjectMapper mapper = new ObjectMapper();

properties = mapper.readValue(jsonString, new TypeReference<Map<String, String>>() {

});

} catch (IOException e) {

log.error(e.getMessage(), e);

}

To convert JSON to a more complicated object, for example a list List<MyObject> we would need to construct a collection type. API Doc JavaType.class

JavaType collectionType = mapper.getTypeFactory().constructCollectionType(List.class, MyObject.class);
List<MyObject> readValue = mapper.readValue(jsonString, collectionType);

1 2	JavaType collectionType = mapper.getTypeFactory().constructCollectionType(List.class, MyObject.class); List<MyObject> readValue = mapper.readValue(jsonString, collectionType);

For more information refer to the official Jackson Main Page.

@TestExecutionListeners is not present for class

January 3, 2014January 3, 2014 by Dmitri Slinkov | 1 Comment

As I was writing and running some integration tests I came across this error message.

DEBUG: org.springframework.test.context.TestContextManager - @TestExecutionListeners is not present for class [class ...]: using defaults.
DEBUG: org.springframework.test.context.TestContextManager - Could not instantiate default TestExecutionListener class [org.springframework.test.context.web.ServletTestExecutionListener]. Specify custom listener classes or make the default listener classes available.

DEBUG: org.springframework.test.context.TestContextManager - @TestExecutionListeners is not present for class [class ...]: using defaults.

DEBUG: org.springframework.test.context.TestContextManager - Could not instantiate default TestExecutionListener class [org.springframework.test.context.web.ServletTestExecutionListener]. Specify custom listener classes or make the default listener classes available.

Turns out I did not have @Test annotation on a method I was trying to run as a JUnit test. The interesting thing I only got that error when other methods in that test class were annotated with @Test. If there were no other methods it would throw a "No runnable methods exception". Completely trivial but I did not immediately connect the error message to the actual cause.

TestExecutionListener API Doc

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Display deployed version on your maven powered Spring application

Setting up WordPress on Amazon Lightsail

Check whether you have multiple ContextLoader* definitions in your web.xml!

7 Essential Eclipse Plugins

Spring’s @Transactional Important Points

1. Spring provides support for transactions using AOP Proxies

Converting JSON object to a generic class with Jackson

@TestExecutionListeners is not present for class