Demo project for Spring Boot, Ehcache, h2 db, delegation with BeanPostProcessor.
In this application delegation is used along with spring boot by overriding BeanPostProcessor interface's postProcessAfterInitialization() method.

@EnableJpaRepositories annotation is used on main class to Enable H2 DB related configuration, which will read properties from application.properties file to perform read operations.

EhcacheDelegationConfig class is used as delegation, which wraps SuperHeroRepository inside EhcacheSuperHeroRepositoryImpl if EhCache bean is configured in classpath.

Cache will be checked if and only if Ehcache bean is configured else EhcacheSuperHeroRepositoryImpl will be skipped.

• Java
• Spring Boot
• Maven
• H2 Database
• Lombok
• Ehcache

• IntelliJ IDEA or Eclipse/STS (or any preferred IDE) with embedded Maven
• Maven (version >= 3.6.0)
• Postman (or any RESTful API testing tool), even any web browser would work as we are going to test readonly REST APIs.

GOTO > ~/absolute-path-to-directory/spring-boot-h2-ehcache-delegation
and try below command in terminal

mvn spring-boot:run it will run application as spring boot application

or

mvn clean install it will build application and create jar file under target directory

Run jar file from below path with given command

java -jar ~/path-to-spring-boot-h2-ehcache-delegation/target/spring-boot-h2-ehcache-delegation-0.0.1-SNAPSHOT.jar

Or

run main method from SpringBootH2EhcacheDelegationApplication.java as spring boot application.

1. Maven Dependencies

   Need to add below JPA & H2 dependencies to enable H2 DB related config in pom.xml.
   Lombok dependency is to get rid of boiler-plate code.
   AOP for aspect related features to print logs using @LogObjectBefore & @LogObjectAfter custom annotations.
   Ehcache for enabling in memory custom Ehcache in the application

   <dependency>
       <groupId>org.springframework.boot</groupId>
       <artifactId>spring-boot-starter-data-jpa</artifactId>
   </dependency>
   
   <dependency>
       <groupId>org.springframework.boot</groupId>
       <artifactId>spring-boot-starter-aop</artifactId>
   </dependency>
   
   <dependency>
       <groupId>com.h2database</groupId>
       <artifactId>h2</artifactId>
       <scope>runtime</scope>
   </dependency>
   
   <dependency>
       <groupId>org.projectlombok</groupId>
       <artifactId>lombok</artifactId>
       <optional>true</optional>
   </dependency>   
   
   <dependency>
       <groupId>net.sf.ehcache</groupId>
       <artifactId>ehcache</artifactId>
   </dependency>
   

2. Properties file

   Reading H2 DB related properties from application.properties file and configuring JPA connection factory for H2 database.

   src/main/resources/application.properties

   spring.datasource.url=jdbc:h2:mem:sampledb
   spring.datasource.driverClassName=org.h2.Driver
   spring.datasource.username=sa
   spring.datasource.password=password
   spring.jpa.database-platform=org.hibernate.dialect.H2Dialect
   
   spring.h2.console.enabled=true 
   
   # This will enable circular dependencies which we are using for delegation
   spring.main.allow-circular-references=true
   

3. Model class

   Below model classes which we will store in H2 DB and perform read operations.
   SuperHero.java

   @Data
   @AllArgsConstructor
   @NoArgsConstructor
   @Builder
   @Entity
   @Table
   public class SuperHero implements Serializable {
   
       @Id
       @GeneratedValue
       private int id;
   
       private String name;
       private String superName;
       private String profession;
       private int age;
       private boolean canFly;
   
       // Constructor, Getter and Setter
   }
   

   The annotations are added in this controller to print log smartly with the help of Aspect (LoggerAspect class from aop package).

   @LogObjectBefore - annotation created to print log before method execution
   @LogObjectAfter - annotation created to print the returned value from method

4. Read operation for Super Heroes

   In SuperHeroController.java class, we have exposed 3 endpoints for basic READ operations

   • GET All Super Heroes
   • GET by ID
   • GET by ID IN

   @RequiredArgsConstructor
   @RestController
   @RequestMapping("/super-heroes")
   public class SuperHeroController {
   
       private final SuperHeroService superHeroService;
       
       @LogObjectAfter
       @GetMapping
       public ResponseEntity<List<?>> findAll();
   
       @LogObjectBefore
       @LogObjectAfter
       @GetMapping("/{id}")
       public ResponseEntity<?> findById(@PathVariable Integer id);
   
       @LogObjectBefore
       @LogObjectAfter
       @GetMapping("/in")
       public ResponseEntity<List<?>> findByIdIn(@RequestParam List<Integer> ids);
   }
   

   In SuperHeroServiceImpl.java, we are injecting SuperHeroRepository interface using constructor injection for read operation.

   In SuperHeroRepository.java, we are extending JpaRepository<Class, ID> interface which has read related methods by putting query inside @Query annotation, which we are overriding in EhcacheSuperHeroRepositoryImpl class.

   public interface SuperHeroRepository extends JpaRepository<SuperHero, Integer> {
    
        @Query("SELECT s FROM SuperHero s WHERE s.id = ?1")
        Optional<SuperHero> findById(Integer id);
   
        @Query("SELECT s FROM SuperHero s")
        List<SuperHero> findAll();
   
        @Query("SELECT s FROM SuperHero s where s.id in (:ids)")
        List<SuperHero> findByIdIn(List<Integer> ids);
   }
   

5. Ehcache Delegation Config class

   This is the most important class in spring delegation pattern and in this application also. This will override the default behaviour of spring JPA by returning EhcacheSuperHeroReoisitoryImpl class implementation by overriding BeanPostProcessor's postProcessAfterInitialization() method instead of JPA repository if and only if Ehcache bean is present in classpath else default JPA repository will be in a picture.
   In simple words if Ehcache bean is configured in application then first SuperHero data will be found in cache and then in DB and will be added in cache after fetching from DB.
   @ConditionalOnBean(Ehcache.class) will check Ehcache bean is present in application only then this call will get configured.
   @ConditionalOnClass(Ehcache.class) will check Ehcache class is present in application only then this call will get configured. This will prevent from getting net.sf.ehcache.Ehcache class not found exception.

   @Slf4j
   @Configuration
   @RequiredArgsConstructor
   @ConditionalOnBean(Ehcache.class)
   @ConditionalOnClass(Ehcache.class)
   public class EhcacheDelegationConfig implements BeanPostProcessor {
   
       private final Ehcache ehcache;
   
       @Override
       public Object postProcessAfterInitialization(Object bean, String beanName) throws BeansException {
   
           if (bean instanceof SuperHeroRepository) {
               log.info("*** Ehcache delegation with EhcacheSuperHeroRepositoryImpl class implementation in use ***");
               return new EhcacheSuperHeroRepositoryImpl((SuperHeroRepository) bean, ehcache);
           }
           return bean;
       }
   }
   

6. Ehcache Config class (Optional)

   This is optional class. If we configure this class in application then Ehcache will be enabled, also EhcacheSuperHeroRepositoryImpl's implementation will be in use.
   If we delete this class or comment the content of it then BeanPostProcessor's postProcessAfterInitialization() method will not delegate the EhcacheSuperHeroRepositoryImpl class and requests will directly reach to DB without checking any cache data.

   @EnableCaching
   @Configuration
   public class EhcacheCacheConfig {
   
       @Bean
       public CacheManager getCustomCacheManager() {
           CacheManager cacheManager = CacheManager.create(getEhCacheConfiguration());
           cacheManager.addCache(ehcache());
           return cacheManager;
       }
   
       @Primary
       @Bean
       public Ehcache ehcache() {
           CacheConfiguration cacheConfig = new CacheConfiguration("my-cache", 1000)
                   .memoryStoreEvictionPolicy(MemoryStoreEvictionPolicy.LRU)
                   .eternal(false)
                   .timeToLiveSeconds(3600)
                   .timeToIdleSeconds(3600);
           return new Cache(cacheConfig);
       }
   
       private net.sf.ehcache.config.Configuration getEhCacheConfiguration() {
           net.sf.ehcache.config.Configuration configuration = new net.sf.ehcache.config.Configuration();
           DiskStoreConfiguration diskStoreConfiguration = new DiskStoreConfiguration();
           diskStoreConfiguration.setPath("java.io.tmpdir");
           configuration.addDiskStore(diskStoreConfiguration);
           return configuration;
       }
   }
   

• Super Hero CRUD Operations

  GET Mapping http://localhost:8080/super-heroes - Get all Super Heroes from DB. If found no record, will add mock data from MockDataHelperUtil class to DB.

  GET Mapping http://localhost:8080/super-heroes/1 - Get Super Hero by ID

  GET Mapping http://localhost:8080/super-heroes/in?ids=1,2,3 - Get all Super Heroes who has id 1, 2 or 3