摘要：用于應用發生不可調整的異常，導致應用退出的情況。聲明如下使用方式通過來注冊，當應用發生異常時，會調用每個的實現類。取值前后正負不同，取最新相同，取絕對值大的。

在線上環境中，應用可能因為一些異常而終止，我們如果需要及時找到原因，根據 exit code 來定位，是個很好的途徑。 spring boot 為開發者提供了相關的接口，方便開發者通過異常類型來定義自己的 exit code：ExitCodeGenerator 和 ExitCodeExceptionMapper.

用于主動退出應用，在 SpringApplication.exit(org.springframework.context.ApplicationContext,ExitCodeGenerator..)中用到，該方法會尋找所有的 ExitCodeGenerator 的 Bean, 也會用到方法中的入參對象。

ExitCodeGenerator 聲明如下：

@FunctionalInterface
public interface ExitCodeGenerator {

    /**
     * Returns the exit code that should be returned from the application.
     * @return the exit code.
     */
    int getExitCode();

}

System.exit(SpringApplication.exit(SpringApplication.run(DemoApplication.class, args)));

@SpringBootApplication
public class DemoApplication{

    @Bean
    public ExitCoder getTestExitCoder(){
        return new ExitCoder();
    }

    @Bean
    public ExitCoder1 getTestExitCoder1(){
        return new ExitCoder1();
    }


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

import org.springframework.boot.ExitCodeGenerator;


public class ExitCoder implements ExitCodeGenerator {
    @Override
    public int getExitCode() {
        System.out.println("get exit code from class: ExitCoder");
        return 222;
    }
}

import org.springframework.boot.ExitCodeGenerator;

public class ExitCoder1 implements ExitCodeGenerator {
    @Override
    public int getExitCode() {
        System.out.println("get exit code from class: ExitCoder1");
        return 221;
    }
}

輸出為

2019-03-21 21:52:55.802  INFO 44627 --- [           main] com.example.exitcode.DemoApplication     : Starting DemoApplication on lei.local with PID 44627 (/Users/lei/own/projects/java_learning/spring_boot_learning/blog/5exitcode/exitcode/out/production/classes started by lei in /Users/lei/own/projects/java_learning/spring_boot_learning/blog/5exitcode/exitcode)
2019-03-21 21:52:55.806  INFO 44627 --- [           main] com.example.exitcode.DemoApplication     : No active profile set, falling back to default profiles: default
2019-03-21 21:52:56.339  INFO 44627 --- [           main] com.example.exitcode.DemoApplication     : Started DemoApplication in 15.901 seconds (JVM running for 21.676)
get exit code from class: ExitCoder
get exit code from class: ExitCoder1
Disconnected from the target VM, address: "127.0.0.1:50873", transport: "socket"

Process finished with exit code 222

從上面可以看到，以 exit code 最大的為最終值。

用于應用發生不可調整的異常，導致應用退出的情況。聲明如下：

@FunctionalInterface
public interface ExitCodeExceptionMapper {

    /**
     * Returns the exit code that should be returned from the application.
     * @param exception the exception causing the application to exit
     * @return the exit code or {@code 0}.
     */
    int getExitCode(Throwable exception);

}

通過 Bean 來注冊，當應用發生異常時，會調用每個ExitCodeExceptionMapper 的實現類。這里，我們可以根據異常類型來設置自己的 exit code:

@SpringBootApplication
public class DemoApplication{
    @Bean
    public DemoExitCodeExceptionMapper getExitCodeMapper(){
        return new DemoExitCodeExceptionMapper();
    }

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


    @Bean
    CommandLineRunner showUsers() {
        return args -> {throw new Exception("xxxxx");};
    }
}

public class DemoExitCodeExceptionMapper implements ExitCodeExceptionMapper{
    /**
     * Returns the exit code that should be returned from the application.
     * @param exception the exception causing the application to exit
     * @return the exit code or {@code 0}.
     */
    @Override
    public int getExitCode(Throwable exception){
        System.out.println("exit cod xxxx" + exception.getMessage());
        if(exception.getCause().getMessage().equals("sdf")){
            return 254;
        }
        return 243;
    }
}

運行輸出為：

2019-03-21 22:13:34.261  INFO 45049 --- [           main] com.example.exitcode.DemoApplication     : Started DemoApplication in 15.816 seconds (JVM running for 21.521)
exit cod xxxxFailed to execute CommandLineRunner
2019-03-21 22:13:38.797  INFO 45049 --- [           main] ConditionEvaluationReportLoggingListener : 

Error starting ApplicationContext. To display the conditions report re-run your application with "debug" enabled.
2019-03-21 22:13:38.845 ERROR 45049 --- [           main] o.s.boot.SpringApplication               : Application run failed

java.lang.IllegalStateException: Failed to execute CommandLineRunner
    at org.springframework.boot.SpringApplication.callRunner(SpringApplication.java:816) [spring-boot-2.1.3.RELEASE.jar:2.1.3.RELEASE]
    at org.springframework.boot.SpringApplication.callRunners(SpringApplication.java:797) [spring-boot-2.1.3.RELEASE.jar:2.1.3.RELEASE]
    at org.springframework.boot.SpringApplication.run(SpringApplication.java:324) [spring-boot-2.1.3.RELEASE.jar:2.1.3.RELEASE]
    at org.springframework.boot.SpringApplication.run(SpringApplication.java:1260) [spring-boot-2.1.3.RELEASE.jar:2.1.3.RELEASE]
    at org.springframework.boot.SpringApplication.run(SpringApplication.java:1248) [spring-boot-2.1.3.RELEASE.jar:2.1.3.RELEASE]
    at com.example.exitcode.DemoApplication.main(DemoApplication.java:31) [classes/:na]
Caused by: java.lang.Exception: xxxxx
    at com.example.exitcode.DemoApplication.lambda$showUsers$0(DemoApplication.java:37) [classes/:na]
    at org.springframework.boot.SpringApplication.callRunner(SpringApplication.java:813) [spring-boot-2.1.3.RELEASE.jar:2.1.3.RELEASE]
    ... 5 common frames omitted

Disconnected from the target VM, address: "127.0.0.1:51237", transport: "socket"

Process finished with exit code 243

public int getExitCode() {
    int exitCode = 0;
    for (ExitCodeGenerator generator : this.generators) {
        try {
            int value = generator.getExitCode();
            if (value > 0 && value > exitCode || value < 0 && value < exitCode) {
                exitCode = value;
            }
        }
        catch (Exception ex) {
            exitCode = (exitCode != 0) ? exitCode : 1;
            ex.printStackTrace();
        }
    }
    return exitCode;
}

第一個: 直接取第一個的值。

2....n-1,n: n 和n-1 正負不同，取 n, 相同，取絕對值大的。

主動調用SpringApplication.exit 方法使用ExitCodeGenerator ,可以通過 Bean注冊，也可通過傳值。

應用異常退出使用 ExitCodeExceptionMapper, 只能通過 Bean 注冊使用。

取值：
前后正負不同，取最新, 相同，取絕對值大的。