Following the online docs for adding jersey to Sring boot, it appears I just need to include the following package
spring-boot-starter-jersey
Actually, it states that Spring Boot provides automatic configuration by including this package.
Where can I find out what exactly is happening?
If I don't add this package then what is NOT configured?
I tried searching for the package in google but got no specific explanation only saying that it automatically configures, but configures what?
I would like to know a little more of what is happening behind the scenes.
All of the auto-configuration code for all that Spring Boot supports is in the spring-boot-autoconfigure module. If you look through the packages, you will see a jersey package.
The "starter" modules generally do not have any code (of course unless it is a third-party module). How it works is that the code in the auto-configuration has some annotations that are #ConditionalOnXxx, where the condition be anything from a class being on the classpath. If this class is not available, then the auto-configuration will not take place. That's pretty much all adding the jersey starter module does: it adds the jersey dependencies so that the auto-configurer will applied.
Now what exactly is being auto-configured specifically for Jersey? Check out the source for the JerseyAutoConfiguration. Basically what you are going to see is your ResourceConfig being injected into the configurer. From that ResourceConfig, it creates Jersey's ServletContainer (which is the main entry point for Jersey.
Then, depending on our properties configuration, either a FilterRegistrationBean or a ServletRegistrationBean is created as a Spring bean, wrapping Jersey's ServletContainer. Jersey can be created as a Servlet or a Servlet Filter. Whichever one we configure we be used.
And that's it for the Jersey configuration. Spring Boot will get a servlet container (e.g. Tomcat, Jetty) from some other auto-configuration, and take the Filter/ServletRegistrationBean and add the Servlet/Filter to that servlet container.
Also, not really that important, but the auto-configuration also give us some Jackson configuration helpers. For example, instead of configuring our own ContextResolver, we can just configure an ObjectMapper Spring bean.
That's really all you get. It's nothing so spectacular that you couldn't just do it yourself without depending on the auto-configuration.
I would recommend learning a little more about how Spring Boot works to provide the automatic configuration in general.
A good reference is the spring boot reference guide. http://docs.spring.io/spring-boot/docs/1.5.3.RELEASE/reference/htmlsingle/#common-application-properties
You can look at the common application properties in appendix A to see what all spring allows you to configure out of the box.
The Reference guide also gives a high level of what "spring-boot-starter-jersey" if you search for it on the page.
They also have a few samples that you can go through and debug to follow along if that is a way for you to learn.
Hope that gives you a starting point for learning!
Related
I understand how SpringBoot saves time in other respects such as having an embedded server and starter dependencies, but how does SpringBoot reduce boiler plate code needed for an application?
Thanks
Spring Boot brings a ton of autoconfiguration classes, which create beans with default configurations, that would have been created by the developer themselves previously. An example would be beans for database access. You would have created a datasource, maybe a JdbcTemplate, connection pool etc. Now those beans are created with autoconfiguration (example: https://github.com/spring-projects/spring-boot/blob/master/spring-boot-project/spring-boot-autoconfigure/src/main/java/org/springframework/boot/autoconfigure/jdbc/DataSourceAutoConfiguration.java), and configuration can be customized through application.properties files.
Spring boot comes with starters and through maven you can search for the required dependency and add it to your project it supports rapid development and below are some key features of spring boot
Removes boilerplate code of application setup by having embedded web
server(Tomcat) and in memory db.
Auto configuration of application context.
Automatic servlet mappings.
Embedded database support(h2)
Automatic controller mapping
You can look at Spring Boot as an opinionated distribution of Spring. It comes with sane defaults and machanisms to hide the boilerplate while still making changes to those defaults possible.
When you use annotations #SpringBootApplication, Spring boot takes care of creating all the beans required for running WebServer and injecting it using its Dependency Injection feature.
#SpringBootApplication is alone equivalent to below three annotations.
#Configuration : You can define your own configuration class to register your beans in application context.
2.#EnableAutoConfiguration : Spring automatically creates beans available on your classs path using this feature.More details are available here.
#ComponentScan : Scans the current and base package where your application class lies.
It Creates ApplicationContext which contains all the necessary beans, ServletWebServerApplicationContext is one such bean created which takes care of initializing and running a WebServer by looking for ServletWebServerFactory bean(provides the webServer) within the ApplicationContext.
There is lot more going on behind the scene. Here is a video which explains it in details.
https://youtu.be/uCE3x4-GQ0k
https://docs.spring.io/spring-boot/docs/current/api/org/springframework/boot/web/servlet/context/ServletWebServerApplicationContext.html
I am a new user of Spring framework. I am facing some confusion in understanding the difference between core spring framework and spring boot. As far as I understand, Spring boot is a framework which performs the initial setup automatically (like Setting up Maven dependencies and downloading the jar files) and comes with an embedded Tomcat server which makes it ready to deploy in just one click., Whereas, Spring MVC requires manual setup. All the tutorials that I watched for core spring show bean configuration using bean factory which configures the beans using a .XML file. In Spring boot, this bean configuration file is absent. My question is, what is the use of this bean configuration file? I did not find any legitimate use of this file in making a REST service with spring. I didn't see any use of the Application Context, Bean Factory in creating web application. Can someone point out how can bean factory be used in Spring web apps? Is there any fundamental difference between core spring and spring boot other than the additional components?
The Spring application context is essentially the "pool" of beans (service objects, which include controllers, converters, data-access objects, and so on) and related information that define an application; I recommend the reference introduction. In theory, you can get complicated with the context setup and have hierarchical organization and such, but in most real-world cases you just have a single plain context.
Inside this context you need to install all of the beans that provide the logic for your application. There are several possible ways to do this, but the two main ways are by providing XML files with have directives like bean (define an individual bean) or component-scan (automatically search for classes with certain annotations, including #Controller) and by using Java classes annotated with #Configuration, which can use annotations and #Bean methods.
The XML style is generally older, and newer applications mostly use Java configuration, but both provide entries that are collected into the context, and you can use both simultaneously. However, in any application, you have to provide some way of getting the registration started, and you will typically have one "root" XML file or configuration class that then imports other XML files and/or configuration classes. In a legacy web.xml-based application, you specify this in your servlet configuration file.
Spring Boot is, as you said, essentially a collection of ready-to-go configuration classes along with a mechanism for automatically detecting configurations and activating them. Even this requires a configuration root, though! This is the #EnableAutoConfiguration instruction, frequently used through its composite #SpringBootApplication. The application context and configuration mechanisms work normally once Boot finds them and pulls them in. Spring knows where to get started because you give it an explicit instruction to build a context starting with that entry point, usually with SpringApplication.run(MyApplication.class, args).
The embedded-server configuration just happens to be a particular set of configuration that is really useful and comes with one of the Boot starter packages. There's nothing there that you couldn't do in a non-Boot application.
I am assessing whether spring-boot and how I could migrate to using it.
One question I have is whether a project that uses spring boot can be converted easily back to a regular spring project which uses the traditional spring configuration files if that is required. This would be useful in my mind for a few reasons.
1) merging with legacy projects, because as I have read moving from legacy spring to spring-boot is somewhat tedious.
2) Obtaining a view of the spring application context file and webapp configuration files to understand what the actual configurations being used are.
Another question I have is regarding the lack of application-context file, is there a way to have some kind of hybrid where there is still an application-context file that can be seen? Part of my concern is that spring-boot auto configures components without us knowing and learning how they are configured and work together.
Spring Boot provides auto-configuration.
When #SpringBootApplication is encountered, it triggers a search of the CLASSPATH for a file called META-INF/spring.factories which is just a regular text file that enumerates a list of Java configuration classes. Java configuration was introduced in 2006 and then merged into Spring 3 in 2009. So it's been around for a long time. These Java configuration classes define beans in the same way that XML does. Each class is annotated with #Configuration and therein you find beans defined using methods (factory methods, or provider methods) whose return value is managed and exposed via Spring. Each such provider method is annotated with #Bean. This tells Spring to consider the method and its return value the definition of the bean.
Spring Boot tries to launch all the Java configurations it sees enumerated in that text file. It tries to launch RabbitAutoConfiguration.class, which in turn provides beans for connecting to RabbitMQ and so on. Of course, you don't want those beans in certain cases, so Spring Boot takes advantage of Spring framework 4's #Conditional mechanism to conditionally register those beans only if certain conditions are met: is a type on the CLASSPATH, is a property exposed through the environment, has there been another bean of the same type defined by the user, etc. Spring boot uses these to only create the RabbitMQ-specific beans if, for example, the dependencies that you would get from org.springframework.boot:spring-boot-starter-amqp are on the CLASSPATH. It also considers that the user may have provided a different implementation of RabbitTemplate in some othe rbean definition (be it XML or Java configuration) so it uses that if it's there.
These java configuration classes are the same sort of Java configuration classes you would write without Spring Boot. BUT... WHY? 80% of the time, the auto-configuration that Spring Boot provides is going to be as good or better than the configuration you would write yourself. There are only so many ways to configure Spring MVC, Spring Data, Spring Batch, etc., and the wager you take using Spring Boot is that the leaders and engineers on those various projects can provide the most sensible 80%-case configuration that you probably don't care to write, anyway.
So, yes you could use Spring Boot with existing applications, but you'd have to move those existing applications to Spring 4 (which is easy to do if you're using the spring-boot-starter-* dependencies) to take advantage of #Conditional. Spring Boot prefers: NO configuration, Java configuration, XML configuration, in that order.
If you have an existing application, I'd do the following:
find out what dependencies you can remove from your Gradle/Maven build and just have taken care of for you with the various spring-boot-starter- dependencies.
add #SpringBootApplication to a root component class. Eg, if your package is a.b.c, put a class Application in a.Application and annotate that with #SpringBootApplication
You can run it as a Servlet 3 application or in an embedded servlet container. It might be easier to just run in a standard servlet container as you take baby steps. Go to http://start.spring.io and make sure to choose war in the packaging drop down. Copy the ServletInitializer class and the specification from the pom.xml to ensure that your application is a .war, not a .jar. Then, once everything works on Spring Boot, rm -rf the Initializer and then revert the Maven build to a simpler .jar using the Spring Boot plugin for extra win.
If your application has lots of existing configuration, import it using #Import(OldConfig.class) or #ImportResource("old-config.xml") on the a.Application configuration class. The auto-configuration will kick in but it will see, for example, that you may have already defined a DataSource so it'll plug that in in the right places. What I like to do now is just start the application up, see if everything's OK, then start removing code in my old Java or XML configuration. Don't remove your business code, but remove things related to turning on parts of Spring. Anything that looks like #Enable..* or ..:annotation-driven/>. Restart and verify things still work. The goal is to let Spring Boot do as much of the heavy lifting as possible. Viewing the source is very handy here so you can see what Spring Boot will try to do for you. Spring Boot will log information on what #Conditional conditions evaluated to true for you. Simply provide --Ddebug=true wen you start the application to see the output. You could also export DEBUG=true then restart your IDE to run it as long as the environment variable is ivsible in your shell.
I am no fan of gross over abstractions, And i think Spring has committed a major felony.
But I'm willing to overlook it this time if someone can explain the algorithm behind the 'auto' configuration.
Having a look at spring's own javadocs, It doesn't give much away other than saying that it will intelligently guess what you need and something to do about conditional beans.
Does someone know what algorithm is used to determine what needs to be loaded?
In my experience as a Spring Boot user the basic factors for Spring Boot to decide on what auto-configurations will be enabled are:
1) The classes present on the classpath. For example if RabbitMQ and Spring AMQP classes are present, then the RabbitAutoConfiguration will be enabled. The corresponding annotation is #ConditionalOnClass,
2) The presence or not of user defined beans. For example, if all the Spring Data JPA is present on the classpath, Spring Boot will register a LocalContainerEntityManagerFactoryBean bean only if the user has not already done so. The beans registered by the user will 'override' the default ones. The relevant annotation is #ConditionalOnMissingBean
As #DaveSyer mentions, you can of course use Spring Boot without #EnableAutoConfiguration if you want to include the relevant configuration on your own. Or you could use the less drastic solution of the exclude field of #EnableAutoConfiguration. If for example you want Spring Boot to autoconfigure everything except ActiveMQ, you would use #EnableAutoConfiguration(exclude=ActiveMQAutoConfiguration.class)
In my opinion, there is absolutely no felony here! You can use what you want from Spring Boot. When you don't want something it has to offer, you can easily opt out partially or completely!
Also if you want to get a look under the covers, you can add the property
logging.level.org.springframework.boot=DEBUG
to application.properties and Spring Boot will gladly give a detailed report of what was auto-configured and what wasn't
There is some documentation in the Spring Boot Reference Guide. It's not terribly complicated, and I hardly think it's a felony to just include a bunch of #Configuration that you might have written anyway (because that's all it does). Feel free not to use #EnableAutoConfiguration if you prefer to include the individual configurations individually.
I have application consisting two web apps and EAR level jar files.
One web app (Lets say SPApp) is built using Spring and another using Struts (STApp).
I want to share the Aspect class SystemArchitecture of SPApp as defined here spring aop
6.2.3.3. Sharing common pointcut definitions
in STApp.
I have added SystemArchitecture class in EAR level jar file and gets invoked from SPApp but doesn't execute when STApp is accessed.
So then I moved the aspect class SystemArchitecture inside STApp and surprisingly it worked.
I am not sure what is going wrong when I place SystemArchitecture in EAR level lib.
Please help.
Thanks,
Hanumant
Spring AOP will not work outside spring. The Spring implementation of AOP is based on dynamic proxying where the spring bean factory will proxy advised classes to inject your pointcuts. What you are after is what's called "load time weaving" in where a java agent is used to intercept the ClassLoader and decorate advised classes when they are loaded. This is an AspectJ functionality, not a Spring AOP. Read more here: http://www.eclipse.org/aspectj/doc/released/devguide/ltw.html
Spring and AspectJ can play together as well, but it's limited to spring driven applications: http://static.springsource.org/spring/docs/3.0.0.RC2/spring-framework-reference/html/ch07s08.html
Still, if you want "true" AOP, independant of Spring then you need to go load-time weaving and AspectJ.
EDIT: may have misread your question. You say it works in your struts app only when you put your advise class in the app itself. From that I read that your Struts app is Spring driven as well. It's hard to give an answer without knowing your config. Specifically, web.xml (both), application.xml and your spring configs.