Spring Boot 中初始化資源的幾種方式

假設有這麼一個需求，要求在項目啓動過程當中，完成線程池的初始化，加密證書加載等功能，你會怎樣作？若是沒有想好答案，請接着往下看。今天介紹幾種在Spring Boot中進行資源初始化的方式，幫助你們解決和回答這個問題。

CommandLineRunner

• 定義初始化類 MyCommandLineRunner

• 實現 CommandLineRunner 接口，並實現它的 run() 方法，在該方法中編寫初始化邏輯

• 註冊成 Bean，添加 @Component 註解

• 示例代碼以下：

@Component
 public class MyCommandLineRunner implements CommandLineRunner {
     
 	@Override
 	public void run(String... args) throws Exception {
 		System.out.println("... init resources by implements CommandLineRunner");
 	}
 }

實現了 CommandLineRunner 接口的 Component 會在全部 Spring Beans 初始化完成以後，在 SpringApplication.run() 執行以前完成。下面經過加兩行打印來驗證咱們的測試。

@SpringBootAppliction
public class DemoApplication {
    
	public static void main(String[] args){
		System.out.println("... start SpringApplication.run()");
		SpringAppliction.run(DemoAppliction.class, args);
		System.out.println("... end SpringApplication.run()");
	}
}

​

控制檯打印結果以下。

... start SpringApplication.run()

  .   ____          _            __ _ _
 /\\ / ___'_ __ _ _(_)_ __  __ _ \ \ \ \
( ( )\___ | '_ | '_| | '_ \/ _` | \ \ \ \
 \\/  ___)| |_)| | | | | || (_| |  ) ) ) )
  '  |____| .__|_| |_|_| |_\__, | / / / /
 =========|_|==============|___/=/_/_/_/
 :: Spring Boot ::        (v2.2.5.RELEASE)

2020-03-09 10:37:13.537  INFO 13456 --- [           main] com.example.demo.DemoApplication         : Starting DemoApplication on DESKTOP-9P44RJ5 with PID 13456 (D:\work\workspace\demo\target\classes started by 78787 in D:\work\workspace\demo)
2020-03-09 10:37:13.539  INFO 13456 --- [           main] com.example.demo.DemoApplication         : No active profile set, falling back to default profiles: default
2020-03-09 10:37:14.131  INFO 13456 --- [           main] o.s.b.w.embedded.tomcat.TomcatWebServer  : Tomcat initialized with port(s): 8080 (http)
2020-03-09 10:37:14.137  INFO 13456 --- [           main] o.apache.catalina.core.StandardService   : Starting service [Tomcat]
2020-03-09 10:37:14.141  INFO 13456 --- [           main] org.apache.catalina.core.StandardEngine  : Starting Servlet engine: [Apache Tomcat/9.0.31]
2020-03-09 10:37:14.203  INFO 13456 --- [           main] o.a.c.c.C.[Tomcat].[localhost].[/]       : Initializing Spring embedded WebApplicationContext
2020-03-09 10:37:14.203  INFO 13456 --- [           main] o.s.web.context.ContextLoader            : Root WebApplicationContext: initialization completed in 638 ms
2020-03-09 10:37:14.307  INFO 13456 --- [           main] o.s.s.concurrent.ThreadPoolTaskExecutor  : Initializing ExecutorService 'applicationTaskExecutor'
2020-03-09 10:37:14.404  INFO 13456 --- [           main] o.s.b.w.embedded.tomcat.TomcatWebServer  : Tomcat started on port(s): 8080 (http) with context path ''
2020-03-09 10:37:14.406  INFO 13456 --- [           main] com.example.demo.DemoApplication         : Started DemoApplication in 1.094 seconds (JVM running for 1.754)
... init resources by implements CommandLineRunner
... end SpringApplication.run()

ApplicationRunner

• 定義初始化類 MyApplicationRunner
• 實現 ApplicationRunner 接口，並實現它的 run() 方法，在該方法中編寫初始化邏輯
• 註冊成 Bean，添加 @Component 註解便可
• 示例代碼以下

@Component
public class MyApplicationRunner implements ApplicationRunner {
 
    @Override
    public void run(ApplicationArguments applicationArguments) throws Exception {
        System.out.println("...init resources by implements ApplicationRunner");
    }
}

​

​ 能夠看到，經過實現 ApplicationRunner 接口，和經過實現 CommandLineRunner 接口均可以完成項目的初始化操做，實現相同的效果。二者以前惟一的區別是 run() 方法中自帶的形參不一樣，在 CommandLineRunner 中只是簡單的 String... args 形參，而 ApplicationRunner 中是包含了 ApplicationArguments 對象，能夠幫助獲取更豐富的項目信息。

public interface ApplicationArguments {
    String[] getSourceArgs();

    Set<String> getOptionNames();

    boolean containsOption(String name);

    List<String> getOptionValues(String name);

    List<String> getNonOptionArgs();
}

@Order

​ 若是項目中既有實現了 ApplicationRunner 接口的初始化類，又有實現了 CommandLineRunner 接口的初始化類，那麼會是哪個先執行呢？測試告訴咱們，答案是實現了 ApplicationRunner 接口的初始化類先執行，我想這點卻是不須要你們過度取關注爲何，但若是須要改變兩個初始化類之間的默認執行順序，那麼使用 @Order 註解就能夠幫助咱們解決這個問題。

@Retention(RetentionPolicy.RUNTIME)
@Target({ElementType.TYPE, ElementType.METHOD, ElementType.FIELD})
@Documented
public @interface Order {
    int value() default 2147483647;
}

@Component
@Order(1)
public class MyCommandLineRunner implements CommandLineRunner {
    @Override
    public void run(String... args) throws Exception {
        System.out.println("... init resources by implements CommandLineRunner");
    }
}

@Component
@Order(2)
public class MyApplicationRunner implements ApplicationRunner {
    @Override
    public void run(ApplicationArguments args) throws Exception {
        System.out.println("... init resources by implements ApplicationRunner");
    }
}

​ 最終，控制檯打印以下，經過控制檯輸出咱們發現，@Order 註解值越小，該初始化類也就越早執行。

....(省略部分代碼)
2020-03-09 10:41:34.858  INFO 20256 --- [           main] com.example.demo.DemoApplication         : Started DemoApplication in 1.151 seconds (JVM running for 1.821)
... init resources by implements CommandLineRunner
... init resources by implements ApplicationRunner
... end SpringAppliction.run()

@PostConstruct

​ 使用 @PostConstruct 註解一樣能夠幫助咱們完成資源的初始化操做，前提是這些初始化操做不須要依賴與其餘 Spring Bean 的初始化工做。

@Documented
@Retention (RUNTIME)
@Target(METHOD)
public @interface PostConstruct {
}

@Component
public class Test {

    @PostConstruct
    public void testPostConstruct(){
        System.out.println("... post construct ");
    }
}

​ 啓動項目，控制檯打印以下。

2020-03-09 10:49:34.229  INFO 24996 --- [           main] o.s.web.context.ContextLoader            : Root WebApplicationContext: initialization completed in 603 ms
... post construct 
2020-03-09 10:49:34.322  INFO 24996 --- [           main] o.s.s.concurrent.ThreadPoolTaskExecutor  : Initializing ExecutorService 'applicationTaskExecutor'
2020-03-09 10:49:34.412  INFO 24996 --- [           main] o.s.b.w.embedded.tomcat.TomcatWebServer  : Tomcat started on port(s): 8080 (http) with context path ''
2020-03-09 10:49:34.414  INFO 24996 --- [           main] com.example.demo.DemoApplication         : Started DemoApplication in 1.031 seconds (JVM running for 1.697)
... init resources by implements CommandLineRunner
... init resources by implements ApplicationRunner
... end SpringAppliction.run()

文末小結

​ 綜上，使用 @PostConstruct 註解進行初始化操做的順序是最快的，前提是這些操做不能依賴於其它Bean的初始化完成。經過添加 @Order 註解，咱們能夠改變同層級之間不一樣Bean的加載順序。