Spring Framework Manual

This is a guide of basics feature of Spring Framework. All the materials are from the official guide, and some of the codes and examples are created myself, which is not so authoritative. However, hope you find something useful in the passage.

Overview

Structure Your Code

Technically, Spring Boot does not require any specific code layout to work. However, there are some best practices that help:

• Your main application class:

• It is generally recommend that you locate your main application class in a root package above other
  classes. The @SpringBootApplication annotation is often placed on your main class, and it implicitly
  defines a base “search package” for certain items

• For example, the follwoing structure:

   com
   +- example
       +- myapplication
           +- Application.java
           |
           +- customer
           |   +- Customer.java
           |   +- CustomerController.java
           |   +- CustomerService.java
           |   +- CustomerRepository.java
           |
           +- order
               +- Order.java
               +- OrderController.java
               +- OrderService.java
               +- OrderRepository.java

• Your primary configuration class

• Although it is possible to use SpringApplication with XML sources, we generally recommend that your primary source be a single @Configuration class. Usually the class that defines the main method is a good candidate as the primary @Configuration.

• You can also import configuration classes:

• The @Import annotation can be used to import additional configuration classes. Alternatively, you can use @ComponentScan to automatically pick up all Spring components, including @Configuration classes.

• Import XML Configuration

• If you need to use an XML based configuration, we recommend that you still start with a @Configuration class. You can then use an @ImportResource annotation to load XML configuration files.

• A single @SpringBootApplication annotation can be used to enable three features, that is:

  • @EnableAutoConfiguration: enable Spring Boot’s auto-configuration mechanism, which *imports many xxxAutoConfiguration classes defined by
    @Import(AutoConfigurationImportSelector.class) *(you will see this when you look at the @EnableAutoConfigurations class). More details see the section below.

  • @ComponentScan: enable @Component scan on the package where the application is located (see the best practices)

  • @Configuration: allow to register extra beans in the context or import additional configuration classes
    SO, the @SpringBootApplication annotation is equivalent to using @Configuration, @EnableAutoConfiguration, and @ComponentScan with their default attributes, as shown in the following example:

  package com.example.myapplication;
  
  import org.springframework.boot.SpringApplication;
  import org.springframework.boot.autoconfigure.SpringBootApplication;
  
  @SpringBootApplication // same as @Configuration @EnableAutoConfiguration @ComponentScan
  public class Application {
  
      public static void main(String[] args) {
          SpringApplication.run(Application.class, args);
      }
  
  }
  

Auro-Configuration

You can also choose to use the Spring Boot auto-configuration, which attempts to automatically configure your Spring application based on the jar dependencies that you have added.

For example, if the H2 database Jar is present in the classpath and we have not configured any beans related to the database manually, the Spring Boot’s auto-configuration feature automatically configures it in the project.

To enable auto-configuration, you need to add either @EnableAutoConfiguration or @SpringBootApplication annotations to one of your @Configuration classes.

EnableAutoConfiguration under the hood:

• When you click into the @EnableAutoConfiguration annotation, you will see that it imports the AutoConfigurationImportSelector.class.

• Within that class, it uses the method List configurations = SpringFactoriesLoader.loadFactoryNames(..) to return to your configurations that will be imported into your application.

• When you click into that method loadFactoryNames(), you will see the line: classLoader.getResources(FACTORIES_RESOURCE_LOCATION), where that Factories_RESOURCE_LOCATION points to: \.m2\repository\org\springframework\boot\spring-boot-autoconfigure\2.3.0.RELEASE\spring-boot-autoconfigure-2.3.0.RELEASE.jar!\META-INF\spring.factories

• The xxxAutoConfiguration classes specified in that spring.factories file will be loaded into your application, if conditions specified in those classes are fulfilled (e.g. @ConditionalOnMissingProperties(..))

• And it is those xxxAutoConfiguration classes that configures your Spring Boot Application by reading/injecting values from their respective .properties file. This means that you can configure those settings in your own application.properties file with the correct prefix

Springboot Configuration

Configuration Files

Springboot apply a global configuration file and this file name is fixed.

• application.properties
• syntactic structure: key = value
• application.yml
• syntactic structure: key: value

The function of configuration file: modify the default value in Springboot configuration, because Springboot has allocated these values in the background.

YAML Configuration File

TAML is the short for “YAML Ain’t a Markup Language”. It is a configuration language which can assign value to objects.

For example, to assgn values to the object Person:

package com.danny.pojo;

import org.springframework.boot.context.properties.ConfigurationProperties;
import org.springframework.stereotype.Component;

import java.util.Date;
import java.util.List;
import java.util.Map;

@Component
@ConfigurationProperties(prefix = "person")

//@ConfigurationProperties: To map the values in configuration file to the component.
// And tell SpringBoot to combine the properties in this class with the values in the configuration files
// The parameter prefix = "Person" : To correspond the values in configuration file with the properties of Person object

public class Person {
//    person:
//    name: Danny
//    happy: false
//    birth: 2020/7/11
//    maps: {k1: v1,k2 v2}
//    list:
//     - code
//     - music
//     - sport
//    dog=Dog{name='cat', age=3}
    private String name;
    private Integer age;
    private Boolean happy;
    private Date birth;
    private Map<String,Object> maps;
    private List<Object> lists;
    private Dog dog;

    public Person() {
    }

    public Person(String name, Integer age, Boolean happy, Date birth, Map<String, Object> maps, List<Object> lists, Dog dog) {
        this.name = name;
        this.age = age;
        this.happy = happy;
        this.birth = birth;
        this.maps = maps;
        this.lists = lists;
        this.dog = dog;
    }

    public String getName() {
        return name;
    }

    public Integer getAge() {
        return age;
    }

    public Boolean getHappy() {
        return happy;
    }

    public Date getBirth() {
        return birth;
    }

    public Map<String, Object> getMaps() {
        return maps;
    }

    public List<Object> getLists() {
        return lists;
    }

    public Dog getDog() {
        return dog;
    }

    public void setName(String name) {
        this.name = name;
    }

    public void setAge(Integer age) {
        this.age = age;
    }

    public void setHappy(Boolean happy) {
        this.happy = happy;
    }

    public void setBirth(Date birth) {
        this.birth = birth;
    }

    public void setMaps(Map<String, Object> maps) {
        this.maps = maps;
    }

    public void setLists(List<Object> lists) {
        this.lists = lists;
    }

    public void setDog(Dog dog) {
        this.dog = dog;
    }

    @Override
    public String toString() {
        return "Person{" +
                "name='" + name + '\'' +
                ", age=" + age +
                ", happy=" + happy +
                ", birth=" + birth +
                ", maps=" + maps +
                ", lists=" + lists +
                ", dog=" + dog +
                '}';
    }
}

The configuration YAML file is demonstrated as the following:

person:
  name: Danny
  happy: false
  birth: 2020/7/11
  maps: {k1: v1,k2 v2}
  list:
    - code
    - music
    - sport
  dog:
    name: cat
    age: 3

In the test class:

package com.danny;

import com.danny.pojo.Dog;
import com.danny.pojo.Person;
import org.junit.jupiter.api.Test;
import org.springframework.beans.factory.annotation.Autowired;
import org.springframework.boot.test.context.SpringBootTest;

@SpringBootTest
class Demo01HelloworldApplicationTests {

    @Autowired
    private Person person;
    @Test
    void contextLoads() {
        System.out.println(person);
    }
}

Ths output in the following :

  .   ____          _            __ _ _
 /\\ / ___'_ __ _ _(_)_ __  __ _ \ \ \ \
( ( )\___ | '_ | '_| | '_ \/ _` | \ \ \ \
 \\/  ___)| |_)| | | | | || (_| |  ) ) ) )
  '  |____| .__|_| |_|_| |_\__, | / / / /
 =========|_|==============|___/=/_/_/_/
 :: Spring Boot ::        (v2.3.2.RELEASE)

2020-07-28 16:21:27.176  INFO 16576 --- [           main] c.d.Demo01HelloworldApplicationTests     : Starting Demo01HelloworldApplicationTests on 举世无双的暗影精灵 with PID 16576 (started by pc in D:\zwh52\coursework\SpringBoot\Springbootdemo)
2020-07-28 16:21:27.178  INFO 16576 --- [           main] c.d.Demo01HelloworldApplicationTests     : No active profile set, falling back to default profiles: default
2020-07-28 16:21:27.777  INFO 16576 --- [           main] c.d.Demo01HelloworldApplicationTests     : Started Demo01HelloworldApplicationTests in 1.175 seconds (JVM running for 2.81)

Person{name='Danny', age=null, happy=false, birth=Sat Jul 11 00:00:00 CST 2020, maps={k1=v1, k2v2=}, lists=null, dog=Dog{name='cat', age=3}}

Process finished with exit code 0

We can find that the vlues in the configuration YAMl file has been assigned to the object.
If we only require a single value in the configuration file, we can use @Value, if there are many values to be assigned, then application.yml configuration file is the first choice.

Multiple Configuration

When developing programs, it usually requires multiple running enviornments, such as developing configuration, testing configuration.

We can use multi-document module in YAML.

For example, the YAML can be seperated by ---:

person:
  name: Danny
  happy: false
  birth: 2020/7/11
  maps: {k1: v1,k2 v2}
  list:
    - code
    - music
    - sport
  dog:
    name: cat
    age: 3
dog:
  name: cat
  age: 3
spring:
  profiles:
    active: dev      //this indicate that the executing configuration is "dev" part

 ---
person:
  name: Frank
  happy: true
  birth: 2020/7/22
  maps: {k1: v1,k2 v2}
  list:
    - Math
    - music
    - Reading
  dog:
    name: Dig
    age: 10
dog:
  name: Dig
  age: 10
spring:
  profiles: dev 

Then, the output is the following:

  .   ____          _            __ _ _
 /\\ / ___'_ __ _ _(_)_ __  __ _ \ \ \ \
( ( )\___ | '_ | '_| | '_ \/ _` | \ \ \ \
 \\/  ___)| |_)| | | | | || (_| |  ) ) ) )
  '  |____| .__|_| |_|_| |_\__, | / / / /
 =========|_|==============|___/=/_/_/_/
 :: Spring Boot ::        (v2.3.2.RELEASE)

2020-07-28 17:04:00.623  INFO 19092 --- [           main] c.d.Demo01HelloworldApplicationTests     : Starting Demo01HelloworldApplicationTests on 举世无双的暗影精灵 with PID 19092 (started by pc in D:\zwh52\coursework\SpringBoot\Springbootdemo)
2020-07-28 17:04:00.624  INFO 19092 --- [           main] c.d.Demo01HelloworldApplicationTests     : The following profiles are active: dev
2020-07-28 17:04:01.012  INFO 19092 --- [           main] c.d.Demo01HelloworldApplicationTests     : Started Demo01HelloworldApplicationTests in 0.77 seconds (JVM running for 1.605)

Person{name='Frank', age=null, happy=true, birth=Wed Jul 22 00:00:00 CST 2020, maps={k1=v1, k2v2=}, lists=null, dog=Dog{name='Dig', age=10}}
Dog{name='Dig', age=10}


Process finished with exit code 0

The principle of autoConfiguration:

1. SpringBoot will load huge amount of autoConfigration classes.
2. We can check if the function we need is in default autoConfiguration classes.
3. If the componet we need is in the classes, we do not need to configurate the component mannully.
4. When adding components in the autoConfigurated classes, it will obtain attributes in the properties classes. We only need to point out the properties of the values of these properties.

xxxAutoConfiguration: auto-generated configuration class.

xxxProperties: The relating attributes in the encapsulated configuration files.

Injecting Property Values

Basically, once the application.properties or other customized application.properties has been loaded, you can inject values with the placeholder ${}.

In the main application where you run your SpringApplication, you can inject them easily with @Value("${}"). For example:

inside application.properties:

app.my.name=jason

then in your main application:

@Value("${app.my.name}")
private String appName;

However, in some other places, besides the main application, you might need to specify your application.properties file again before injecting the @Value. For example, in a @Configuration file at some other place in your project hierarchy:

@Configuration
@PropertySource("classpath:application.properties")
public class MainConfig {
    @Bean
    public ProfileManager profileManager(@Autowired Environment env, @Value("${app.testname}") String s) {
        return new ProfileManager(env,s);
    }
}

You can also use this to calculate expressions, which will require #{}instead of ${} for @Value(). For example:

@Value(#{app.my.age*2})
private int age;

Binding with a Customized yml or properties file

You can also also specify a customized properties file for constructor injection or property bindings. The basic annotations and syntax are the same as above, except that you need to specify that customized yml or properties file with @PropertySource(). For example:

@PropertySource(value="classpath: customizedPropertyFile")
@ConfigurationProperties(prefix="person1")
public class Person
    ...

SpringBoot Features

The SpringApplication class provides a convenient way to bootstrap a Spring application that is started from a main() method. In many situations, you can delegate to the static SpringApplication.run method, as shown in the following example:

public static void main(String[] args) {
	SpringApplication.run(MySpringConfiguration.class, args);
}

When your application starts, you should see something similar to the following output:

  .   ____          _            __ _ _
 /\\ / ___'_ __ _ _(_)_ __  __ _ \ \ \ \
( ( )\___ | '_ | '_| | '_ \/ _` | \ \ \ \
 \\/  ___)| |_)| | | | | || (_| |  ) ) ) )
  '  |____| .__|_| |_|_| |_\__, | / / / /
 =========|_|==============|___/=/_/_/_/
 :: Spring Boot ::        (v2.3.0.RELEASE)

2020-06-16 15:35:35.216  INFO 47936 --- [  restartedMain] App                                      : Starting App on XY-Laptop with PID 47936 (E:\Maven Workspace\testSpringBoot\target\classes started by 26238 in E:\Maven Workspace\testSpringBoot)
2020-06-16 15:35:35.218  INFO 47936 --- [  restartedMain] App                                      : No active profile set, falling back to default profiles: default
2020-06-16 15:35:35.248  INFO 47936 --- [  restartedMain] .e.DevToolsPropertyDefaultsPostProcessor : Devtools property defaults active! Set 'spring.devtools.add-properties' to 'false' to disable
2020-06-16 15:35:35.248  INFO 47936 --- [  restartedMain] .e.DevToolsPropertyDefaultsPostProcessor : For additional web related logging consider setting the 'logging.level.web' property to 'DEBUG'
2020-06-16 15:35:36.345  INFO 47936 --- [  restartedMain] o.s.b.w.embedded.tomcat.TomcatWebServer  : Tomcat initialized with port(s): 8080 (http)
2020-06-16 15:35:36.351  INFO 47936 --- [  restartedMain] o.apache.catalina.core.StandardService   : Starting service [Tomcat]
2020-06-16 15:35:36.351  INFO 47936 --- [  restartedMain] org.apache.catalina.core.StandardEngine  : Starting Servlet engine: [Apache Tomcat/9.0.35]
2020-06-16 15:35:36.399  INFO 47936 --- [  restartedMain] o.a.c.c.C.[Tomcat].[localhost].[/]       : Initializing Spring embedded WebApplicationContext
2020-06-16 15:35:36.399  INFO 47936 --- [  restartedMain] o.s.web.context.ContextLoader            : Root WebApplicationContext: initialization completed in 1151 ms
2020-06-16 15:35:36.504  INFO 47936 --- [  restartedMain] o.s.s.concurrent.ThreadPoolTaskExecutor  : Initializing ExecutorService 'applicationTaskExecutor'
2020-06-16 15:35:36.588  INFO 47936 --- [  restartedMain] o.s.b.d.a.OptionalLiveReloadServer       : LiveReload server is running on port 35729
2020-06-16 15:35:36.623  INFO 47936 --- [  restartedMain] o.s.b.w.embedded.tomcat.TomcatWebServer  : Tomcat started on port(s): 8080 (http) with context path ''
2020-06-16 15:35:36.630  INFO 47936 --- [  restartedMain] App                                      : Started App in 1.652 seconds (JVM running for 1.947)

Startup Failures

If your application fails to start, registered FailureAnalyzers get a chance to provide a dedicated error message and a concrete action to fix the problem. For instance, if you start a web application on port 8080 and that port is already in use, you should see something similar to the following message:

***************************
APPLICATION FAILED TO START
***************************
Description:
Embedded servlet container failed to start. Port 8080 was already in use.
Action:
Identify and stop the process that's listening on port 8080 or configure this
application to listen on another port.

If no failure analyzers are able to handle the exception, you can still display the full conditions report to better understand what went wrong. To do so, you need to enable the debug property or enable DEBUG logging for org.springframework.boot.autoconfigure.logging.ConditionEvaluationReportLoggingListener.

For instance, if you are running your application by using java -jar, you can enable the debug
property as follows:

$ java -jar myproject-0.0.1-SNAPSHOT.jar --debug

SpringBoot Web

Spring Boot is well suited for web application development. You can create a self-contained HTTP server by using embedded Tomcat, Jetty, Undertow, or Netty. Most web applications use the springboot-starter-web module to get up and running quickly. You can also choose to build reactive web applications by using the spring-boot-starter-webflux module.

Spring Web MVC Framework

The Spring Web MVC framework (often referred to as simply “Spring MVC”) is a rich “model view controller” web framework. Spring MVC lets you create special @Controller or @RestController beans to handle incoming HTTP requests (@Controller would be returning data that is being rendered by view, and @RestController would be returning raw data). Methods in your controller are mapped to HTTP by using @RequestMapping annotations.

The following code shows a typical @RestController that serves JSON data (by default):

@RestController
@RequestMapping(value="/users")
public class MyRestController {
	@RequestMapping(value="/{user}", method=RequestMethod.GET)
	public User getUser(@PathVariable Long user) {	// objects returned are turned into JSON format
		// ...
	}

	@RequestMapping(value="/{user}/customers", method=RequestMethod.GET)
	List<Customer> getUserCustomers(@PathVariable Long user) {
		// ...
	}

	@RequestMapping(value="/{user}", method=RequestMethod.DELETE)
	public User deleteUser(@PathVariable Long user) {
		// ...
	}
}

Spring MVC is part of the core Spring Framework, and detailed information is available in the reference documentation. There are also several guides that cover Spring MVC available at spring.io/guides.

Spring Boot Framework

Since Spring Boot has all those AutoConfiguration classes doing the configuration work for us, it is much simpler and faster for us to develop a web application using Spring Boot. In summary, all we need to do is:

• Create a Spring Boot project (if you use Spring Initialzr, you will see that it is based on Spring Boot) with your desired dependencies
• Spring Boot will load all those AutoConfiguration files, which you can further customize using application.properties or application.yml
  Write your own program
• The most important step is to understand the mechanism behind AutoConfiguration, which uses the xxxAutoConfiguration classes that has properties injected (identified with a certain prefix).

Spring Boot Web Application Project Structure

WebJars

WebJars is simply taking the concept of a JAR and applying it to client-side libraries or resources. For example, the jQuery library may be packaged as a JAR and made available to your Spring MVC application. There are several benefits to using WebJars, including support for Java build tools such as Gradle and Maven.

For Spring Boot to load and later package your web contents correctly, you need to add your contents at a correct place. If you look into your WebMVCAutoConfiguration class, you will see the following method:

public void addResourceHandlers(ResourceHandlerRegistry registry) {
    if (!this.resourceProperties.isAddMappings()) {
        logger.debug("Default resource handling disabled");
    } else {
        Duration cachePeriod = this.resourceProperties.getCache().getPeriod();
        CacheControl cacheControl = this.resourceProperties.getCache().getCachecontrol().toHttpCacheControl();
        if (!registry.hasMappingForPattern("/webjars/**")) {
            this.customizeResourceHandlerRegistration(registry.addResourceHandler(new String[] "/webjars/**"}).addResourceLocations(new String[]{"classpath:/META-INF/resources/webjars/"}).setCachePeriod(this.getSeconds(cachePeriod)).setCacheControl(cacheControl));
        }

        String staticPathPattern = this.mvcProperties.getStaticPathPattern();
        if (!registry.hasMappingForPattern(staticPathPattern)) {
            this.customizeResourceHandlerRegistration(registry.addResourceHandler(new String[]{staticPathPattern}).addResourceLocations(WebMvcAutoConfiguration.getResourceLocations(this.resourceProperties.getStaticLocations())).setCachePeriod(this.getSeconds(cachePeriod)).setCacheControl(cacheControl));
        }

    }
}

So you see that all your web contents need to be placed in classpath:/META-INF/resources/webjars/ directory. This means that within your project, any file stored under /META-INF/resources/webjars/ can be visited by localhost:/webjars/

For example, on the https://www.webjars.org/ you can choose which webjars to import into your project by copying the following to your pom file.

<dependency>
    <groupId>org.webjarsgroupId>
    <artifactId>bootstrapartifactId>
    <version>4.0.0version>
dependency>

Then in your external dependency, you can see that it also has the structure mentioned above, and you can visit its resources with localhost:8080/webjars/bootstrap/4.0.0/webjars-requirejs.js, for example. Then you will see that js file displayed:

/*global requirejs */

// Ensure any request for this webjar brings in jQuery.
requirejs.config({
  paths: { 
    "bootstrap": webjars.path("bootstrap", "js/bootstrap"),
    "bootstrap-css": webjars.path("bootstrap", "css/bootstrap")  
  },
  shim: { "bootstrap": [ "jquery" ] }
});

Static contents

In the same method where you find those webjar structure, you can also find where you should store your static contents:

String staticPathPattern = this.mvcProperties.getStaticPathPattern();
                if (!registry.hasMappingForPattern(staticPathPattern)) {
                    this.customizeResourceHandlerRegistration(registry.addResourceHandler(new String[]{staticPathPattern}).addResourceLocations(WebMvcAutoConfiguration.getResourceLocations(this.resourceProperties.getStaticLocations())).setCachePeriod(this.getSeconds(cachePeriod)).setCacheControl(cacheControl));
                }

which finds your resource under the following directories:

"classpath:/META-INF/resources/", 
"classpath:/resources/", 
"classpath:/static/", 
"classpath:/public/"
"/" (project root path)

This means that if a request is not handled, you will be redirected to the static files under those locations.

For example, if you have an html page under the following directory:

Then you can visit the html by localhost:8080/signin/index.html.

Welcome Page

Welcome page is basically the home page of your application. Spring Boot also has a default mapping defined in the WebMvcAutoConfiguration class:

@Bean
public WelcomePageHandlerMapping welcomePageHandlerMapping(ApplicationContext applicationContext, FormattingConversionService mvcConversionService, ResourceUrlProvider mvcResourceUrlProvider) {
    WelcomePageHandlerMapping welcomePageHandlerMapping = new WelcomePageHandlerMapping(new TemplateAvailabilityProviders(applicationContext), applicationContext, this.getWelcomePage(), this.mvcProperties.getStaticPathPattern());
    welcomePageHandlerMapping.setInterceptors(this.getInterceptors(mvcConversionService, mvcResourceUrlProvider));
    welcomePageHandlerMapping.setCorsConfigurations(this.getCorsConfigurations());
    return welcomePageHandlerMapping;
}

This in the end will look for index.html below all static content directories mentioned above, and maps to /.**

For example, if you have the following structure:

Then you can visit this index.html by localhost:8080/

Configure Your Web Properties

If you want to customize those locations, you can also use the spring.resources.xxx to modify the default paths scanned.

This works because in the ResoucrProperties class, Spring Boot defines locations for scanning static resources as shown below:

@ConfigurationProperties(
    prefix = "spring.resources",
    ignoreUnknownFields = false
)
public class ResourceProperties {
    private static final String[] CLASSPATH_RESOURCE_LOCATIONS = new String[]{"classpath:/META-INF/resources/", "classpath:/resources/", "classpath:/static/", "classpath:/public/"};

This means you can change those locations by specifying in your application.properties or applications.yml:

spring.resources.static-locations=classpath:/hello/, classpath:/example

Banner page and Web icon cumstomization

There are four static resources :

• public
• static
• template
• resources

You can put your index.html in either of these files.

To set customized web icon, you can put your onw ico in the public resource, Then, set the spring.mvc.favicon.enable=false in the application.properties.

Importing Thymeleaf

Since we are using Spring Boot with Maven, it is easy to include thymeleaf into your project. If you have already inherited the parent spring-boot-starter, then you just need to include the following dependency in your pom:

<dependency>
    <groupId>org.springframework.bootgroupId>
    <artifactId>spring-boot-starter-thymeleafartifactId>
dependency>
 <dependency>
    <groupId>org.thymeleafgroupId>
    <artifactId>thymeleaf-spring5artifactId> 
dependency>
<dependency>
    <groupId>org.thymeleaf.extrasgroupId>
     <artifactId>thymeleaf-extras-java8timeartifactId>
dependency>

However, this uses the version 2.3.1 (specified in the parent pom). It is recommended to use thymeleaf 3 or above for more functionalities. To do this, you need to configure the properties in your pom.

<properties>
    
	<thymeleaf.version>3.0.11.RELEASEthymeleaf.version>
    <thymeleaf-layout-dialect.version>2.1.1thymeleaf-layout-dialect.version>
properties>

Using Thymeleaf

The simplest example of using Thymeleaf would be to return an html page upon a request.

If we look into the ThymeleafProperties class (located in SpringBootAutoConfigure jar, under the folder thymeleaf), you will see:

@ConfigurationProperties(
    prefix = "spring.thymeleaf"
)
public class ThymeleafProperties {
    private static final Charset DEFAULT_ENCODING;
    public static final String DEFAULT_PREFIX = "classpath:/templates/";
    public static final String DEFAULT_SUFFIX = ".html";
    private boolean checkTemplate = true;
    private boolean checkTemplateLocation = true;
    private String prefix = "classpath:/templates/";
    private String suffix = ".html";
    private String mode = "HTML";
    ...

where

public static final String DEFAULT_PREFIX = "classpath:/templates/";
public static final String DEFAULT_SUFFIX = ".html";

means that Thymeleaf scans through classpath:/templates/ for files ending with .html to be rendered.

Now, to hand you back a html page based on request using Thymeleaf, all you need to do is add a html page with the same return result as your html in your templates folder:

For example, if you have:

Then your handler could look like:

@Controller
public class HTMLRequests {
    @RequestMapping("/getHtmlPage")
    public String testHTMLRequest(){
        return "request1";  // this has to be the same as the html file name
    }
}

• Note:
  Now you should not use a @RestController, since it will automatically use the @ResponceBody which will render the page by Spring Boot itself instead of by Thymeleaf

For more information, you can follow the official thymeleaf guide.

1. th:xxx

   If you place th: inside any html tag, where could be any HTML property (including text, class, id, etc.), then Thymeleaf will replace the original value with the value you specified. For example, you can replace the text appearing in a div with:

   
   <div th:text="${Hello}">
       Original Text
   div>
   

   and the corresponding Java class could look like:

   @Controller
   public class HTMLRequests {
   @RequestMapping("/getHtmlPage")
   public String testHTMLRequest(Map<String, String> map1){
       map1.put("Hello","Value Retrieved from key Hello");	// the value will be retrieved
       
       return "request1";  // this tells you which html file to return
   }
   }
   

2. Precendence

   Since there is no notion of code precedence in html, if we specified multiple th:xxx within the same tag, collision will happen. To solve this, Thymeleaf defines its own precedence rule:
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3. Thymeleaf Expressions

   • Simple expressions:

     • Variable Expressions: ${…}

       • can be used to read/store objects, and invoking certain methods on them

     • Selection Variable Expressions: *{…}

       • mainly used in combination with th:object, which stores an object in the current scope
       • then using *{…} will select/read a certain attribute of that stored object

     • Message Expressions: #{…}

       • used for internationalization

     • Link URL Expressions: @{…}

       • used for generating dynamic URL addresses
       • for example:

       
       <a href="details.html" th:href="@{http://localhost:8080/gtvg/order/details(orderId=${o.id})}">viewa>
       
       
       <a href="details.html" th:href="@{/order/details(orderId=${o.id})}">viewa>
       

     • Fragment Expressions: ~{…}

       • this will be talked about later

   • Literals:

     • Text literals: ‘one text’ , ‘Another one!’ ,…
     • Number literals: 0 , 34 , 3.0 , 12.3 ,…
     • Boolean literals: true , false
     • Null literal: null
     • Literal tokens: one , sometext , main ,…

   • Text operations:

     • String concatenation: +
     • Literal substitutions: |The name is ${name}|

   • Arithmetic operations:

     • Binary operators: + , - , * , / , %
     • Minus sign (unary operator): -

   • Boolean operations:

     • Binary operators: and , or
     • Boolean negation (unary operator): ! , not

   • Comparisons and equality:

     • Comparators: > , < , >= , <= ( gt , lt , ge , le )
     • Equality operators: == , != ( eq , ne )

   • Conditional operators:

     • If-then: (if) ? (then)
     • If-then-else: (if) ? (then) : (else)
       • same as Java tertiary operation
     • Default: (value) ?: (defaultvalue)

   • Special tokens:

     • No-Operation: _

   (For more information, please visit Chapter 4 of the Thymeleaf Documentation)

   Example:

   In your Java program, you can have:

   @Controller
   public class HTMLRequests {
       @RequestMapping("/getHtmlPage")
       public String testHTMLRequest(Map<String, String> map1, Map<String, Object> map2){
           map1.put("Hello","
   Value Retrieved from key Hello with h3 heading
   ");
   
           String[] names = {"jason","michael","bowen"};
           List users = Arrays.asList(names);
           map2.put("Users",users);
           return "request1";
       }
   }
   

   In your html code, you can have:

   
   <html lang="en" xmlns:th="http://www.thymeleaf.org">    
   <head>
       <meta charset="UTF-8">
       <title>Titletitle>
   head>
   <body>
       <h1>Test Requesth1>
       <p> A paragraph here p>
       
       
       <div th:utext="${Hello}">Original textdiv>
       <hr/>
   
       
       <h5 th:text="${user}" th:each="user:${Users}">h5>
       <hr/>
   
       <h5>
           <span th:each="user:${Users}">[[${user}]] span>
       h5>
   
   body>
   html>
   

   where

   • th:each can be seen as a for loop iteration, that does the work of for element: iterable.
   • [[$]]istheinlineversionofth:text=${}. [($)]istheinlineversionofth:utext="${}"

Internalization

To enable your website to show contents in different languages based on users’ preferences (e.g. browser language used), you need to use internationalization(i18n), which is supported by Spring Boot. (In Spring MVC, this is configured by MessageResouce class. However, in Spring Boot, it is configured automatically with MessageSourceAutoConfiguration.)

In summary, you need to do the following steps:

• Create a new folder under your resource folder, in which you will store all your internationalization properties file.

• Create properties file in the format of .properties, which will be used as the default properties file for that specific page xxx (for example).

• Create other properties file in the format of __.properties. In this way, IntelliJ will recognize them, knowing that you are doing internationalization.

• Now, since IntelliJ recognized them as internationalization files, you can use the Resource Bundle option when editing your properties file.

• After configuring your properties file, you need to specify the base path (e.g. a property under the location resources/i18n/login_en_US.properties have the base path of i18n.login ) of your properties file with spring.messages.basename= in your application.properties.

• Finally, you just need to go to your html page and use the thmeleaf messages expression #{...} (for example, if you had a property named login.title=... in your login_en_US.properties file, you should use #{login.title} to render your message expression text automatically based on your browser language.

For example:

if you have done step 1-4 correctly, you should be working like this:

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Internalization Property Files

Note:

• If you used Chinese in your properties files, and you get unreadable codes, you might need to change your decoding to enable ASCII conversion. For example, you can try the following:

Text Encoding Setting

If you want to achieve the language swapping by a button on your page, you need to use the LocaleResolver class (also inside the MessageResouceAutoConfiguration class).

By default, this works by getting the section Accept-Language in the request header when you send your html request. To override this behavior, you need to write your own LocaleResolver class.

For example,

firstly, in your html page, you would have a button that creates those request with language:

<... th:href="@{/index.html(l="zh_CN")}">中文...>
<... th:href="@{/index.html(l="en_US")}">English...>

In your MyLocaleResolver class, you can write:

public class MyLocaleResolver implements LocaleResolver{
    @Override
    public Locale resolveLocale(HttpServletRequest request){
        String lang = request.getParameter("l"); // getting the information l=zh_Ch from the request, for example
        Locale locale = Locale.getDefault(); // if there is none, then gets the default Locale information
        if(!StringUtils.isEmpty(l)){
            String[] split = l.split("_"); // gets the language and country code
            locale = new Locale(split[0]. split[1]);
        }
        return locale;
    }
    
    @Override 
    public void setLocale(...){
        ...
    }
}

Add this component to your application as a bean. This is because the Spring Boot LocaleResolver is injected with the condition: @ConditionalOnMissingBean, meaning that if we injected this bean, then Spring Boot’s default resolver will not be activated:

// in one of your `@Configuration` class, for example
@Bean
public LocaleResolver localeResolver(){
    return new MyLocaleResolver();
}

Sending and Handling Post Request
To handle a user login, for example, you need to do two parts (in abstract).

First, in your HTML page, you need to send a request (th:action="@{}") with your form, in which you have each field that you need to be set with a specific name=xxx attribute.

Then, you add a handler to with PostController() to map to that request, and gets the value in those fields with @RequestParam("") String xxx.
For example:

In your html page, you can have:

<form class="..." action="..." th:action="@{/user-login}" method="post">
    ...
    <input type="..." class="..." placeholder="..." name="username">
    <input type="..." class="..." placeholder="..." name="password">
    ...
form>

In your Java class, you can have:

@Controller
public class LoginController{
    // same as @RequestMapping(method=RequestMethod.POST)
    @PostMapping(value="/user-login")
    public String login(@RequestParam("username") String username,
                       @RequestParam("password") String password){
        ... // your logic to check those username and passwords
        return "dashboard"	// the page in templates folder that you want to show to logged in users
    }
}

Note:

• If hot swapping is not working (assuming you have enabled it) when you changed those static/templates html code, you should try:
  • add the line spring.thymeleaf.cache=false in your application.properties file
  • use Ctrl+F9 or manually click the build button to rebuild your project

If you want to add the functionality of popping up red text in your html page with Login Failed when your user failed the authentication, you can use the th:if functionality with some small changes to your controller code:

In your controller:

@Controller
public class LoginController{
    // same as @RequestMapping(method=RequestMethod.POST)
    @PostMapping(value="/user-login")
    public String login(@RequestParam("username") String username,
                       @RequestParam("password") String password
                       Map<String, String> errmsg){
        ... // your logic to check those username and passwords
        if(failed to authenticate){
            errmsg.put("msg","Login Failed");
            return "login" // your original page
        }
        return "dashboard"	// the page in templates folder that you want to show to logged in users
    }
}

Then in your html code, you can have:

<p style="color: red" th:text="${msg}" th:if="not #strings.isEmpty(msg)">p>

Spring Boot MVC Extensions

Spring Boot provides auto-configuration for Spring MVC that works well with most applications. The auto-configuration adds the following features on top of Spring’s defaults:

• Inclusion of ContentNegotiatingViewResolver and BeanNameViewResolver beans.
• Support for serving static resources, including support for WebJars (covered before)).
• Automatic registration of Converter, GenericConverter, and Formatter beans.
• For example, converting an object to json format
• Support for HttpMessageConverters.
• For example, converts and handles http request
• Automatic regis tration of MessageCodesResolver (covered later in this document).
• Static index.html support (covered before)
• Custom Favicon support
• Automatic use of a ConfigurableWebBindingInitializer bean.

If you want to keep those Spring Boot MVC customizations and make more MVC customizations (interceptors, formatters, view controllers, and other features), you can add your own @Configuration class of type WebMvcConfigurer but without @EnableWebMvc.

If you want to provide custom instances of RequestMappingHandlerMapping, RequestMappingHandlerAdapter, or ExceptionHandlerExceptionResolver, and still keep the Spring Boot MVC customizations, you can declare a bean of type WebMvcRegistrations and use it to provide custom instances of those components.

If you want to take complete control of Spring MVC, you can add your own @Configuration annotated with @EnableWebMvc, or alternatively add your own @Configuration-annotated DelegatingWebMvcConfiguration as described in the Javadoc of @EnableWebMvc.

Example: Adding a ViewController

If you want to control a certain request to show the output of another request, you can use a ViewController to control the view of a page. However, by default Spring Boot does not have AutoConfiguration classes for that. Therefore, you would need to add your own ViewController by “adding your own @Configuration class of type WebMvcConfigurer but without @EnableWebMvc“ (mentioned above).

For instance, to map the request /request to request1 (which you specified as a html page configured by Thymeleaf), you need to write the configuration class:

@Configuration
public class myMVCExtensionConfig implements WebMvcConfigurer {
    @Override
    public void addViewControllers(ViewControllerRegistry registry) {
        registry.addViewController("/request").setViewName("request1");
    }
}

Mechanism

Since WebMvcAutoConfiguration is the AutoConfiguration class for Spring MVC, we can look into that class to understand those extension mechanisms. Within that class, you will see:

@Import({WebMvcAutoConfiguration.EnableWebMvcConfiguration.class})
@EnableConfigurationProperties({WebMvcProperties.class, ResourceProperties.class})
@Order(0)
public static class WebMvcAutoConfigurationAdapter implements WebMvcConfigurer {
    ...
}

Which imports WebMvcAutoConfiguration.EnableWebMvcConfiguration.class, and that class extends DelegatingWebMvcConfiguration by having the following signature:

public static class EnableWebMvcConfiguration extends DelegatingWebMvcConfiguration implements ResourceLoaderAware {

Finally, in the parent class, you will see:

@Autowired(
    required = false
)	// since it is AutoWired, those configures are obtained from the IoC container
public void setConfigurers(List<WebMvcConfigurer> configurers) {
    if (!CollectionUtils.isEmpty(configurers)) {
        this.configurers.addWebMvcConfigurers(configurers);
    }

}

protected void addViewControllers(ViewControllerRegistry registry) {
    this.configurers.addViewControllers(registry);
}

And finally if we trace the configureViewResolvers method, we will see:

public void addViewControllers(ViewControllerRegistry registry) {
    Iterator var2 = this.delegates.iterator();

    while(var2.hasNext()) {
        WebMvcConfigurer delegate = (WebMvcConfigurer)var2.next();
        delegate.addViewControllers(registry);	// calls the method addViewControllers on each WebMvcConfigurer class
        										// and add them to the registry object
    }

}

This is why we needed to extend WebMvcConfigurer class for extending its functionality, add calling the method addViewControllers() in the above example to add our ViewController.