《重构之美》之五

使用面向对象语言进行过程式设计的例子，可谓俯拾皆是。看这段代码： 

public class SyncExecutor {      public void executeSync() {          syncSchools();          syncGrades();          syncFaculties();      }  }

这段代码很清晰，分别执行了对学校、年级与教师信息的同步。一目了然，似乎没有什么问题。然而，如果深入阅读各个同步子方法，就会发现某种坏味道，那就是重复代码。 

private void syncSchools() {      List<School> sourceSchools = getSourceSchools();      List<School> targetSchools = new ArrayList<School>();      List<String> sourceSchoolCodes = getSchoolCodes(sourceSchools);      Map<String,School> targetSchoolWithCodeMapping = schoolService.getSchoolWithCodeMapping(sourceSchoolCodes);      for (School sourceSchool:sourceSchools) {          String schoolCode = sourceSchool.getSchoolCode();          School targetSchool = targetSchoolWithCodeMapping.get(schoolcode);          if (targetSchool == null) {              targetSchool = new School(                  sourceSchool.getSchoolCode(),                  sourceSchool.getSchoolName(),                  sourceSchool.getProvinceCode(),                  sourceSchool.getSchoolAddress(),                  sourceSchool.getSchoolZip(),                  sourceSchool.getSchoolTel());          } else if (isCover) {              targetSchool.setSchoolCode(sourceSchool.getSchoolCode());              targetSchool.setSchoolName(sourceSchool.getSchoolName());              targetSchool.setProvinceCode(sourceSchool.getProvinceCode());              targetSchool.setSchoolAddress(sourceSchool.getSchoolAddress());              targetSchool.setSchoolZip(sourceSchool.getSchoolZip());              targetSchool.setSchoolTel(sourceSchool.getSchoolTel());          }              targetSchools.add(targetSchool);      }

    syncService.saveOrUpdate(targetSchools); 

}  private void syncGrades() {      List<Grade> sourceGrades = getSourceGrades();      List<Grade> targetGrades = new ArrayList<Grade>();      List<String> sourceGradeCodes = getGradeCodes(sourceGrades);      Map<String,Grade> targetGradeWithCodeMapping = gradeService.getGradeWithCodeMapping(sourceGradeCodes);      for (Grade sourceGrade:sourceGrades) {          String gradeCode = sourceGrade.getGradeCode();          Grade targetGrade = targetGradeWithCodeMapping.get(gradeCode);          if (targetGrade == null) {              targetGrade = new Grade(                  sourceGrade.getGradeCode(),                 sourceGrade.getName(),                  sourceGrade.getEntranceDay(),                  sourceGrade.getGraduateDay(),                  sourceGrade.getSchoolCode(),  sourceGrade.getSchoolProperty());          } else if (isCover) {              targetGrade.setGradeCode(sourceGrade.getGradeCode());              targetGrade.setName(sourceGrade.getName());              targetGrade.setEntranceDay(sourceGrade.getEntranceDay());              targetGrade.setGraduateDay(sourceGrade.getGraduateDay());              targetGrade.setSchoolCode(sourceGrade.getSchoolCode());              targetGrade.setSchoolProperty(sourceGrade.getSchoolProperty());          }  targetGrades.add(targetGrade);      }

    syncService.saveOrUpdate(targetGrades); 

}

当然，真实的代码更加复杂与混乱，但如果经过一系列重构，例如Rename Method，Extract Method之后，就会变得逐渐清晰，大体结构如上述展示的代码。阅读这样的代码，是否发现各个同步子方法均有似曾相识的感觉呢？究其原因，在于同步的执行逻辑大体相似，换言之，它们具有相似的模板。我们需要改善其结构，实现代码的重用。然而，在方法层面上，我们已很难实现这一点。事实上，当我们在编写同步方法时，已经落入了过程式设计的窠臼。我们首先想到的是执行的过程，而非对象。现在，我们需要将这些执行过程封装为对象，充分地利用继承等机制实现类级别的重用。显然，这里可以运用Form Template Method重构。当然，在此之前，我们还需要运用Extract Superclass，对School、Grade等类进行一系列重构，例如为它们建立共同的父类Entity，提供getCode()方法。并运用Rename Method，将原来各自实体类的相关方法，例如getSchoolCode()、getGradeCode()等，更名为getCode()。

现在，我们需要为同步操作定义一个共同的抽象类DataSynchronizer，然后利用Move Method重构，将原有SyncExecutor的相关代码搬移到DataSynchronizer中: 

public abstract class DataSynchronizer {      public void execute() {          List<Entity> sourceEntities = getSourceEntities();          List<Entity> targetEntities = new ArrayList<Entity>();          List<String> sourceEntityCodes = getEntityCodes(sourceEntities);          Map<String,Entity> targetEntityWithCodeMapping = getEntityWithCodeMapping(sourceEntityCodes);          for (Entity sourceEntity:sourceEntities) {              String entityCode = sourceEntity.getCode();              Entity targetEntity = targetEntityWithCodeMapping.get(entityCode);              if (targetEntity == null) {                  targetGrade = createEntity(sourceEntity);              } else if (isCover) {                  updateEntity(targetEntity,sourceEntity);              }              targetEntities.add(targetEntity);          }

        syncService.saveOrUpdate(targetEntities); 

    }

    protected abstract List<Entity> getSourceEntities(); 

    protected abstract List<String> getEntityCodes(List<Entity> entities);      protected abstract Map<String,Entity> getEntityWithCodeMapping(List<String> entityCodes);      protected abstract Entity createEntity(Entity sourceEntity);      protected abstract void updateEntity(Entity target, Entity source);  }

注意，在获得Entity与Code的Map对象时，我对原有的代码实现进行了封装，因为不同的实体同步类，所要调用的Service对象是不一样的。因此，需要将调用Service相关方法的实现留给子类。现在，只需要定义各个同步类继承DataSynchronizer，重写相关的受保护抽象方法即可： 

public class SchoolSynchronizer extends DataSynchronizer{}  public class GradeSynchronizer extends DataSynchronizer{}  public class FacultySynchronizer extends DataSynchronizer{}

接着，修改SyncExecutor类的实现。为方便调用同步子类的相关方法，我定义了一个Factory Method： 

public class SyncExecutor {      public void executeSync() {          for (DataSynchronizer dataSync:createSynchronizers()) {              dataSync.execute();          }      }

    protected List<DataSynchronizer> createSynchronizers() { 

        List< DataSynchronizer> synchronizers =  new ArrayList&lt; DataSynchronizer();          synchronizers.add(new SchoolSynchronizer());          synchronizers.add(new GradeSynchronizer());  synchronizers.add(new FacultySynchronizer());

return synchronizers; 

    }  }

以真正面向对象的方式来完成上述功能，无论在代码结构、重用性还是扩展性方面，比诸之前的实现，都有了长足的改善。这就是面向对象设计的优雅之处。

纵观整个重构过程，实际上，我在运用Convert Procedural Design to Objects重构时，大量运用了Rename Method、Extract Method、Move Method、Extract Superclass、Form Template Method等重构手法。这是合乎常情的。当我们在对程序进行重构时，往往需要运用各种重构手法，才能达到最终的重构目的。对于大型重构而言，这种特征尤其明显。

本文转自wayfarer51CTO博客，原文链接：http://blog.51cto.com/wayfarer/462523，如需转载请自行联系原作者