使用註解幹掉大量if else和switch

最近正在作工單系統，每種類型工單的操做是類似的可是又有不一樣，好比工單審批是相同的，可是工單啓動是不一樣的，所以天然而然想到把工單的操做放到一個抽象類中，每種工單實現不一樣的操做。

工單服務抽象類ProcessInstanceService

@Service
public abstract class ProcessInstanceService{
    public abstract String start(BaseParamDTO baseParamDTO);
    
    public void approval(BaseParamDTO baseParamDTO, String user) {
        System.out.println("審批經過");
    }
}
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如今有一種擴容工單和一種權限申請工單

@Service
public class ExpansionService extends ProcessInstanceService{
    @Override
    public String start(BaseParamDTO baseParamDTO) {
        System.out.println("擴容");
    }
}

@Service
public class PrivilegeService extends ProcessInstanceService{
    @Override
    public String start(BaseParamDTO baseParamDTO) {
        System.out.println("權限申請");
    }
} 
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前端啓動工單，這個時候須要根據工單類型獲取對應的服務進行操做，所以使用了工廠模式

public enum  WorksheetTypeEnum {

    EXPANSION("擴容", "machine_expansion"),
    PRIVILEGE("登陸權限","privilege_login");
    
    private String cnName;

    private String activitiDefineName;

    WorksheetTypeEnum(String cnName, String activitiDefineName){
        this.cnName = cnName;
        this.activitiDefineName = activitiDefineName;
    }
}

@Component
public class ProcessInstanceFactory {
    @Autowired
    private ExpansionService expansionService;
    
    @Autowired
    private PrivilegeService privilegeService;
    
    public ProcessInstanceService getProcessInstanceService(WorksheetTypeEnum worksheetTypeEnum){
        switch (worksheetTypeEnum){
            case EXPANSION:
                return expansionService;
            case PRIVILEGE:
                return privilegeService;
            default:
                return null;
        }
    }
}
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當前的代碼已經使用枚舉簡化了if else，可是仍然存在問題，當新增一種工單時，ProcessInstanceFactory必須注入新的工單服務，而後添加新的case。那有沒有什麼方法不用修改工廠類直接添加新的工單類型呢，那就是使用註解。

首先建立一個註解

@Target(ElementType.TYPE)
@Retention(RetentionPolicy.RUNTIME)
public @interface ServiceTypeAnnotation {

    /**
     * 工單類型名稱
     * @return
     */
    WorksheetTypeEnum value();
}
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爲每一個工單服務加上註解

@Service
@ServiceTypeAnnotation(value = WorksheetTypeEnum.EXPANSION)
public class ExpansionService extends ProcessInstanceService{}

@Service
@ServiceTypeAnnotation(value = WorksheetTypeEnum.PRIVILEGE)
public class PrivilegeService extends ProcessInstanceService{}
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修改工廠類ProcessInstanceFactory，在初始化的時候經過上下文拿到全部加了註解的類放到map中

@Component
public class ProcessInstanceFactory {

    @Autowired
    private DefaultService defaultService;

    @Autowired
    private ApplicationContext applicationContext;

    private Map<WorksheetTypeEnum, ProcessInstanceService> map = Maps.newConcurrentMap();

    @PostConstruct
    public void init(){
        Map<String, Object> beansWithAnnotation = applicationContext.getBeansWithAnnotation(ServiceTypeAnnotation.class);
        for (Object bean : beansWithAnnotation.values()){
            ServiceTypeAnnotation annotation = bean.getClass().getAnnotation(ServiceTypeAnnotation.class);
            map.put(annotation.value(), (ProcessInstanceService) bean);
        }
    }

    public ProcessInstanceService getProcessInstanceService(WorksheetTypeEnum worksheetTypeEnum){
        ProcessInstanceService processInstanceService = map.get(worksheetTypeEnum);
        if (processInstanceService == null){
            return defaultService;
        } else {
            return processInstanceService;
        }
    }
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這樣若是須要新增一個工單服務，只須要對新的類加上註解就能夠了，如新增一個手工工單

public enum  WorksheetTypeEnum {

    EXPANSION("擴容", "machine_expansion"),
    PRIVILEGE("登陸權限","privilege_login"),
    ASSIGNMENT("手工工單", "assignments");
    
}

@Service
@ServiceTypeAnnotation(value = WorksheetTypeEnum.ASSIGNMENT)
public class AssignmentService extends ProcessInstanceService{}
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