使用注解干掉大量if else和switch

最近正在作工单系统，每种类型工单的操做是类似的可是又有不一样，好比工单审批是相同的，可是工单启动是不一样的，所以天然而然想到把工单的操做放到一个抽象类中，每种工单实现不一样的操做。

工单服务抽象类ProcessInstanceService

@Service
public abstract class ProcessInstanceService{
    public abstract String start(BaseParamDTO baseParamDTO);
    
    public void approval(BaseParamDTO baseParamDTO, String user) {
        System.out.println("审批经过");
    }
}
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如今有一种扩容工单和一种权限申请工单

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

@Service
public class PrivilegeService extends ProcessInstanceService{
    @Override
    public String start(BaseParamDTO baseParamDTO) {
        System.out.println("权限申请");
    }
} 
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前端启动工单，这个时候须要根据工单类型获取对应的服务进行操做，所以使用了工厂模式

public enum  WorksheetTypeEnum {

    EXPANSION("扩容", "machine_expansion"),
    PRIVILEGE("登陆权限","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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当前的代码已经使用枚举简化了if else，可是仍然存在问题，当新增一种工单时，ProcessInstanceFactory必须注入新的工单服务，而后添加新的case。那有没有什么方法不用修改工厂类直接添加新的工单类型呢，那就是使用注解。

首先建立一个注解

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

    /**
     * 工单类型名称
     * @return
     */
    WorksheetTypeEnum value();
}
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为每一个工单服务加上注解

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

@Service
@ServiceTypeAnnotation(value = WorksheetTypeEnum.PRIVILEGE)
public class PrivilegeService extends ProcessInstanceService{}
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修改工厂类ProcessInstanceFactory，在初始化的时候经过上下文拿到全部加了注解的类放到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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这样若是须要新增一个工单服务，只须要对新的类加上注解就能够了，如新增一个手工工单

public enum  WorksheetTypeEnum {

    EXPANSION("扩容", "machine_expansion"),
    PRIVILEGE("登陆权限","privilege_login"),
    ASSIGNMENT("手工工单", "assignments");
    
}

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