Observer概述

Observer - 图1

UML概述

Observer - 图2

玩具代码案例1 - 模式监听一个数据迁移过程中的事件

事件的管理器

  1. package online.javabook.gof.behavioral.patterns6.observer.migrate.provider;
  3. import online.javabook.gof.behavioral.patterns6.observer.migrate.event.MigrateEvent;
  4. import online.javabook.gof.behavioral.patterns6.observer.migrate.listener.api.IMigrateListener;
  5. import online.javabook.gof.behavioral.patterns9.template1.migrate.template.IConnector;
  7. import java.util.ArrayList;
  8. import java.util.List;
  10. public class MigrateListenerProvider {
  12.     private List<IMigrateListener> listeners = new ArrayList<>();
  14.     public void addListener(IMigrateListener listener) {
  15.         listeners.add(listener);
  16.     }
  18.     public void removeListener(IMigrateListener listener) {
  19.         listeners.remove(listener);
  20.     }
  22.     public void onStartMigrate(IConnector sourceConnector, IConnector targetConnector) {
  23.         for (IMigrateListener listener : listeners){
  24.             try {
  25.                 listener.onStartMigrateTask(new MigrateEvent(sourceConnector, targetConnector));
  26.             }catch (Exception e){
  27.                 System.out.println(e);
  28.             }
  29.         }
  30.     }
  32.     public void onPrepMigrateRow(IConnector sourceConnector, IConnector targetConnector, String row) {
  33.         for (IMigrateListener listener : listeners){
  34.             try {
  35.                 listener.onPrepMigrateRecord(new MigrateEvent(sourceConnector, targetConnector, row));
  36.             }catch (Exception e){
  37.                 System.out.println(e);
  38.             }
  39.         }
  40.     }
  42.     public void onPostMigrateRow(IConnector sourceConnector, IConnector targetConnector, String row) {
  43.         for (IMigrateListener listener : listeners){
  44.             try {
  45.                 listener.onPostMigrateRecord(new MigrateEvent(sourceConnector, targetConnector, row));
  46.             }catch (Exception e){
  47.                 System.out.println(e);
  48.             }
  49.         }
  50.     }
  52.     public void onFailMigrateRow(IConnector sourceConnector, IConnector targetConnector, String row, Exception exception) {
  53.         for (IMigrateListener listener : listeners){
  54.             try {
  55.                 listener.onFailMigrateRecord(new MigrateEvent(sourceConnector, targetConnector, row, exception));
  56.             }catch (Exception e){
  57.                 System.out.println(e);
  58.             }
  59.         }
  60.     }
  62.     public void onFinishMigrate(IConnector sourceConnector, IConnector targetConnector) {
  63.         for (IMigrateListener listener : listeners){
  64.             try {
  65.                 listener.onFinishMigrateTask(new MigrateEvent(sourceConnector, targetConnector));
  66.             }catch (Exception e){
  67.                 System.out.println(e);
  68.             }
  69.         }
  70.     }
  72. }

事件的观察者

观察数据迁移任务过程中的每一个时间点发生的事件，这样很好的将额外的业务逻辑从主线逻辑中分离了

  1. package online.javabook.gof.behavioral.patterns6.observer.migrate.listener.api;
  3. import online.javabook.gof.behavioral.patterns6.observer.migrate.event.MigrateEvent;
  5. public interface IMigrateListener {
  7.     void onStartMigrateTask(MigrateEvent event);
  9.     void onPrepMigrateRecord(MigrateEvent event);
  11.     void onPostMigrateRecord(MigrateEvent event);
  13.     void onFailMigrateRecord(MigrateEvent event);
  15.     void onFinishMigrateTask(MigrateEvent event);
  17. }

默认的适配器

在一个观察者模式中往往会配合默认适配器模式的使用

  1. package online.javabook.gof.behavioral.patterns6.observer.migrate.listener.api;
  3. import online.javabook.gof.behavioral.patterns6.observer.migrate.event.MigrateEvent;
  5. public class DefaultMigrateAdapterListener implements IMigrateListener {
  7.     @Override
  8.     public void onStartMigrateTask(MigrateEvent event) {
  10.     }
  12.     @Override
  13.     public void onPrepMigrateRecord(MigrateEvent event) {
  15.     }
  17.     @Override
  18.     public void onPostMigrateRecord(MigrateEvent event) {
  20.     }
  22.     @Override
  23.     public void onFailMigrateRecord(MigrateEvent event) {
  25.     }
  27.     @Override
  28.     public void onFinishMigrateTask(MigrateEvent event) {
  30.     }
  31. }

观察者的实现

MigrateTaskListener

数据迁移任务的开始与结束时的监听器

  1. package online.javabook.gof.behavioral.patterns6.observer.migrate.listener.impl;
  3. import online.javabook.gof.behavioral.patterns6.observer.migrate.event.MigrateEvent;
  4. import online.javabook.gof.behavioral.patterns6.observer.migrate.listener.api.DefaultMigrateAdapterListener;
  6. public class MigrateTaskListener extends DefaultMigrateAdapterListener {
  8.     @Override
  9.     public void onStartMigrateTask(MigrateEvent event) {
  10.         System.out.println("onStartMigrate -> start migrate task");
  11.     }
  13.     @Override
  14.     public void onFinishMigrateTask(MigrateEvent event) {
  15.         System.out.println("onFinishMigrate -> finish migrate task");
  16.     }
  17. }

MigrateRecordListener

数据迁移单条记录时候的监听器

  1. package online.javabook.gof.behavioral.patterns6.observer.migrate.listener.impl;
  3. import online.javabook.gof.behavioral.patterns6.observer.migrate.event.MigrateEvent;
  4. import online.javabook.gof.behavioral.patterns6.observer.migrate.listener.api.DefaultMigrateAdapterListener;
  6. public class MigrateRecordListener extends DefaultMigrateAdapterListener {
  8.     @Override
  9.     public void onPrepMigrateRecord(MigrateEvent event) {
  10.         System.out.println("onPrepMigrateRecord -> migrate row:" + event.getCurrentRow() + " to mysql");
  11.     }
  13.     @Override
  14.     public void onPostMigrateRecord(MigrateEvent event) {
  15.         System.out.println("onPostMigrateRow -> migrate row:" + event.getCurrentRow() + " to mysql");
  16.     }
  17. }

MigrateMessageListener

数据迁移任务在开始与结束时的消息通知监听器

  1. package online.javabook.gof.behavioral.patterns6.observer.migrate.listener.impl;
  3. import online.javabook.gof.behavioral.patterns6.observer.migrate.event.MigrateEvent;
  4. import online.javabook.gof.behavioral.patterns6.observer.migrate.listener.api.DefaultMigrateAdapterListener;
  6. public class MigrateMessageListener extends DefaultMigrateAdapterListener {
  8.     @Override
  9.     public void onStartMigrateTask(MigrateEvent event) {
  10.         System.out.println("onStartMigrate -> start migrate email notification");
  11.     }
  13.     @Override
  14.     public void onFinishMigrateTask(MigrateEvent event) {
  15.         System.out.println("onFinishMigrate -> finish migrate email notification");
  16.     }
  17. }

MigrateExceptionListener

数据迁移记录时候的异常处理

  1. package online.javabook.gof.behavioral.patterns6.observer.migrate.listener.impl;
  3. import online.javabook.gof.behavioral.patterns6.observer.migrate.event.MigrateEvent;
  4. import online.javabook.gof.behavioral.patterns6.observer.migrate.listener.api.DefaultMigrateAdapterListener;
  6. public class MigrateExceptionListener extends DefaultMigrateAdapterListener {
  8.     @Override
  9.     public void onFailMigrateRecord(MigrateEvent event) {
  10.         System.out.println("onFailMigrateRecord -> migrate row:" + event.getCurrentRow() + " to mysql fail:" + event.getException());
  11.     }
  12. }

MigrateLoggerListener

数据迁移中在合适的事件点添加日志

  1. package online.javabook.gof.behavioral.patterns6.observer.migrate.listener.impl;
  3. import online.javabook.gof.behavioral.patterns6.observer.migrate.event.MigrateEvent;
  4. import online.javabook.gof.behavioral.patterns6.observer.migrate.listener.api.DefaultMigrateAdapterListener;
  6. public class MigrateLoggerListener extends DefaultMigrateAdapterListener {
  8.     @Override
  9.     public void onStartMigrateTask(MigrateEvent event) {
  10.         System.out.println("onStartMigrate -> log start migrate task");
  11.     }
  13.     @Override
  14.     public void onPrepMigrateRecord(MigrateEvent event) {
  15.         System.out.println("onPrepMigrateRow -> log prep migrate record:" + event.getCurrentRow());
  16.     }
  18.     @Override
  19.     public void onPostMigrateRecord(MigrateEvent event) {
  20.         System.out.println("onPostMigrateRow -> log post migrate record:" + event.getCurrentRow());
  21.     }
  23.     @Override
  24.     public void onFailMigrateRecord(MigrateEvent event) {
  25.         System.out.println("onFailMigrateRow -> log fail migrate record:" + event.getCurrentRow());
  26.     }
  28.     @Override
  29.     public void onFinishMigrateTask(MigrateEvent event) {
  30.         System.out.println("onFinishMigrate -> log finish migrate task");
  31.     }
  32. }

事件对象

MigrateEvent

事件中往往封装了程序运行时的上下文信息

  1. package online.javabook.gof.behavioral.patterns6.observer.migrate.event;
  3. import online.javabook.gof.behavioral.patterns9.template1.migrate.template.IConnector;
  5. public class MigrateEvent {
  7.     private IConnector sourceConnector;
  9.     private IConnector targetConnector;
  11.     private String currentRow;
  13.     private Exception exception;
  15.     public MigrateEvent(IConnector sourceConnector, IConnector targetConnector) {
  16.         this.sourceConnector = sourceConnector;
  17.         this.targetConnector = targetConnector;
  18.     }
  20.     public MigrateEvent(IConnector sourceConnector, IConnector targetConnector, String currentRow) {
  21.         this.sourceConnector = sourceConnector;
  22.         this.targetConnector = targetConnector;
  23.         this.currentRow = currentRow;
  24.     }
  26.     public MigrateEvent(IConnector sourceConnector, IConnector targetConnector, String currentRow, Exception exception) {
  27.         this.sourceConnector = sourceConnector;
  28.         this.targetConnector = targetConnector;
  29.         this.currentRow = currentRow;
  30.         this.exception = exception;
  31.     }
  32.     public IConnector getSourceConnector() {
  33.         return sourceConnector;
  34.     }
  36.     public IConnector getTargetConnector() {
  37.         return targetConnector;
  38.     }
  40.     public String getCurrentRow() {
  41.         return currentRow;
  42.     }
  44.     public Exception getException() {
  45.         return exception;
  46.     }
  47. }