Spring Cheatsheet

spring-cheatsheet.java

// Return JSON without Jackson mapping classes
@RequestMapping("/users") 
public @ResponseBody Map<String, String> getUsers () {
    
  Map<String, String> map = new HashMap<String, String>();
  map.put("user", "Clark Kent");
    
  return map;
}

// Autowiring multiple Beans of same type into a Java List
@Autowired
private List<MyBean> beans;

// Get a Bean out of the Spring Application Context
ApplicationContext context = new AnnotationConfigApplicationContext(AppConfig.class);
MyBean myFirstBean = context.getBean("myFirstBean", MyBean.class);

// By default all Spring managed Beans are Singletons (@Scope("singleton"))
MyBean bean1 = context.getBean("myBean", MyBean.class);
MyBean bean2 = context.getBean("myBean", MyBean.class);

bean1 == bean2; // => true

// Create a new Bean on each autowiring and each context.getBean() call
@Bean
@Scope("prototype")
public MyBean myBean() {
  return new MyBean();
}

MyBean bean1 = context.getBean("myBean", MyBean.class);
MyBean bean2 = context.getBean("myBean", MyBean.class);

bean1 == bean2; // => false

// In a web app (Spring MVC) there are two more Scopes: "request" and "session"
// It is also possible to define a custom Scope

// Calling a @Bean annotated method multiple times
// result in the exact same instance of the Bean every time it gets called
// as the scope is "singleton" by default
@Configuration
public class MyConfig {
  @Bean
  public MyBean myBean() {
    return new MyBean();
  }

  public void myTestMethod() {
    MyBean myBean1 = myBean();
    MyBean myBean2 = myBean();

    myBean1 == myBean2 // => true
  }
}

// This is achieved by Spring automatically subclassing the MyConfig Class
// and overriding the myBean() method somehow like this:
public class SpringMyConfig extends MyConfig {
  @Override
  public MyBean myBean() {
    if (myBean already in context) {
      return myBean;
    }
    else {
      MyBean myBean = super.myBean();
      context.add(myBean);
      return myBean;
    }
  }
}

// Call a method right after the constructor + setter methods and a method before the destruction of a bean
@Bean(initMethod = "myStart", destroyMethod = "myEnd")
public MyBean myBean() {
  return new MyBean();
}

public class MyBean {
  public void myStart() {
    // do something
  }
  
  public void myEnd() {
    // do something
  }
}

// OR: on the Bean class directly (JSR 330 -> not hardwired to Spring) - Only works with singleton scoped Beans:
public class MyBean {
  @PostConstruct
  public void myStart() {
    // do something
  }
  
  @PreDestroy
  public void myEnd() {
    // do something
  }
}

// @Transactional - manages a transaction manager
//
// Spring doesn't provide any transaction managers itself.
// It provides hooks to them.
//
// Databases have their own transaction managers which will
// be used by Spring.
//
// Spring requests different features of the underlying transaction
// managers since not all provide the same functionality (e.g. Rollback on Timeout)
//
// For @Transactional to work there has to be a Platform Transaction Manager Bean being declared
// It provides the hook into the underlying transactional system.

// Apply @Transactional to all methods of a class (can be overridden with @Transactional on a specific method)
@Transactional
public class MyBean {
  public void doStuff() {
  }

  public void doOtherStuff() {
  }
}

// Properties of the @Transactional Annotation
// isolation - enum for isolation levels
// propagation - enum for propagation levels
// readOnly - boolean to request a readOnly transaction
// timeout - integer for a timout period
// value - String for the name of the transaction manager Bean (Default is: "transactionManager")
// rollbackFor - List of Exception classes which must trigger a rollback
// rollbackForClassName - List of Strings with classnames of Exception classes which must trigger a rollback
// noRollbackFor - List of Exception classes which must not trigger a rollback
// noRollbackForClassName - List of Strings with classnames of Exception classes which must not trigger a rollback

// PlatformTransactionManager - Interface which is implemented by several Transaction Managers
// (e.g. JmsTransactionManager, JpaTransactionManager)
// it has commit() and rollback() as methods

// 3 Steps to achieve transactional behaviour:
// 1. Set @Transactional on a @Service Bean to achieve that each service method is transactional
@Service
@Transactional
public class MyService {
  @Autowired
  private MyRepository myRepo;
}

// 2. In MyConfig declare a transaction manager
// 3. Annotate MyConfig with @EnableTransactionManagement
//    This tells Spring to generate the transactional Proxies
//    The equivalent in XML configuration is <tx:annotation-driven/>
@Configuration
@EnableTransactionManagement
public class MyConfig {
  @Bean(name = "transactionManager")
  @Profile("test")
  public PlatformTransactionManager transactionManagerForTest() {
    return new DataSourceTransactionManager(dataSourceForTest());
  }
  
  @Bean(name = "dataSource", destroyMethod = "shutdown")
  @Profile("test")
  public DataSource dataSourceForTest() {
    return new EmbeddedDatabaseBuilder()
      .generateUniqueName(true)
      .setType(EmbeddedDatabaseType.H2)
      .setScriptEncoding("UTF-8")
      .ignoreFailedDrops(true)
      .addScript("schema.sql")
      .addScripts("data.sql")
      .build();
  }
}