最近寫代碼的過程中，發現業務邏輯很多的時候，總會有很多的 if ... else，很多的 if ...else，有的時候就會丟去某些邏輯。有的時候也會用到swich case 來區分不同類型下執行不同的方法。但是往往這種情況的時候，會顯得一個方法的代碼非常長，不利於代碼擴展，違反開閉原則。爲了破解這樣的嵌套。我們可以使用策略模式來解決。
目錄
需求
假設有一個訂單系統，裏面的一個功能是根據訂單的不同類型作出不同的處理。
傳統實現方式
public class OrderDTO {
private String code;
private BigDecimal price;
/**
* 訂單類型
* 1：普通訂單；
* 2：團購訂單；
* 3：促銷訂單；
*/
private String type;
// ... 省略 get / set ...
}
public interface IOrderService {
/**
* 根據訂單的不同類型作出不同的處理
*
* @param dto 訂單實體
* @return 爲了簡單，返回字符串
*/
String handle(OrderDTO dto);
}
public class OrderServiceImpl implements IOrderService {
@Override
public String handle(OrderDTO dto) {
String type = dto.getType();
if ("1".equals(type)) {
return "處理普通訂單";
} else if ("2".equals(type)) {
return "處理團購訂單";
} else if ("3".equals(type)) {
return "處理促銷訂單";
}
return null;
}
}
策略模式實現
- 修改後實現類
public class OrderServiceV2Impl implements IOrderService {
@Autowired
private HandlerContext handlerContext;
@Override
public String handle(OrderDTO dto) {
AbstractHandler handler = handlerContext.getInstance(dto.getType());
return handler.handle(dto);
}
}
- 分類後的具體業務邏輯
@Component
@HandlerType("1")
public class NormalHandler extends AbstractHandler {
@Override
public String handle(OrderDTO dto) {
return "處理普通訂單";
}
}
@HandlerType("2")
public class GroupHandler extends AbstractHandler {
@Override
public String handle(OrderDTO dto) {
return "處理團購訂單";
}
}
- 自定義註解
@Target({ElementType.TYPE})
@Retention(RetentionPolicy.RUNTIME)
@Documented
@Inherited
public @interface HandlerType {
String value();
}
- 抽象處理器
public abstract class AbstractHandler {
abstract public String handle(OrderDTO dto);
}
- 項目啓動掃描handler入口
@Component
@SuppressWarnings("unchecked")
public class HandlerProcessor implements BeanFactoryPostProcessor {
private static final String HANDLER_PACKAGE = "com.cipher.handler_demo.handler.biz";
/**
* 掃描@HandlerType，初始化HandlerContext，將其註冊到spring容器
*
* @param beanFactory bean工廠
* @see HandlerType
* @see HandlerContext
*/
@Override
public void postProcessBeanFactory(ConfigurableListableBeanFactory beanFactory) throws BeansException {
Map<String, Class> handlerMap = Maps.newHashMapWithExpectedSize(3);
Set<Class<?>> set = ClassScaner.scan(HANDLER_PACKAGE, HandlerType.class);
for (Class clazz : set) {
HandlerType annotation = (HandlerType)clazz.getAnnotation(HandlerType.class);
handlerMap.put(annotation.value(), clazz);
}
// 初始化HandlerContext，將其註冊到spring容器中
HandlerContext context = new HandlerContext(handlerMap);
beanFactory.registerSingleton(HandlerContext.class.getName(), context);
}
}
- 掃描工具類
public class ClassScaner implements ResourceLoaderAware {
private final List<TypeFilter> includeFilters = new LinkedList<TypeFilter>();
private final List<TypeFilter> excludeFilters = new LinkedList<TypeFilter>();
private ResourcePatternResolver resourcePatternResolver = new PathMatchingResourcePatternResolver();
private MetadataReaderFactory metadataReaderFactory = new CachingMetadataReaderFactory(this.resourcePatternResolver);
@SafeVarargs
public static Set<Class<?>> scan(String[] basePackages, Class<? extends Annotation>... annotations) {
ClassScaner cs = new ClassScaner();
if (ArrayUtils.isNotEmpty(annotations)) {
for (Class anno : annotations) {
cs.addIncludeFilter(new AnnotationTypeFilter(anno));
}
}
Set<Class<?>> classes = new HashSet<>();
for (String s : basePackages) {
classes.addAll(cs.doScan(s));
}
return classes;
}
@SafeVarargs
public static Set<Class<?>> scan(String basePackages, Class<? extends Annotation>... annotations) {
return ClassScaner.scan(StringUtils.tokenizeToStringArray(basePackages, ",; \t\n"), annotations);
}
public final ResourceLoader getResourceLoader() {
return this.resourcePatternResolver;
}
@Override
public void setResourceLoader(ResourceLoader resourceLoader) {
this.resourcePatternResolver = ResourcePatternUtils
.getResourcePatternResolver(resourceLoader);
this.metadataReaderFactory = new CachingMetadataReaderFactory(
resourceLoader);
}
public void addIncludeFilter(TypeFilter includeFilter) {
this.includeFilters.add(includeFilter);
}
public void addExcludeFilter(TypeFilter excludeFilter) {
this.excludeFilters.add(0, excludeFilter);
}
public void resetFilters(boolean useDefaultFilters) {
this.includeFilters.clear();
this.excludeFilters.clear();
}
public Set<Class<?>> doScan(String basePackage) {
Set<Class<?>> classes = new HashSet<>();
try {
String packageSearchPath = ResourcePatternResolver.CLASSPATH_ALL_URL_PREFIX
+ org.springframework.util.ClassUtils
.convertClassNameToResourcePath(SystemPropertyUtils
.resolvePlaceholders(basePackage))
+ "/**/*.class";
Resource[] resources = this.resourcePatternResolver
.getResources(packageSearchPath);
for (int i = 0; i < resources.length; i++) {
Resource resource = resources[i];
if (resource.isReadable()) {
MetadataReader metadataReader = this.metadataReaderFactory.getMetadataReader(resource);
if ((includeFilters.size() == 0 && excludeFilters.size() == 0) || matches(metadataReader)) {
try {
classes.add(Class.forName(metadataReader
.getClassMetadata().getClassName()));
} catch (ClassNotFoundException e) {
e.printStackTrace();
}
}
}
}
} catch (IOException ex) {
throw new BeanDefinitionStoreException(
"I/O failure during classpath scanning", ex);
}
return classes;
}
protected boolean matches(MetadataReader metadataReader) throws IOException {
for (TypeFilter tf : this.excludeFilters) {
if (tf.match(metadataReader, this.metadataReaderFactory)) {
return false;
}
}
for (TypeFilter tf : this.includeFilters) {
if (tf.match(metadataReader, this.metadataReaderFactory)) {
return true;
}
}
return false;
}
}
- 根據類型實例化抽象類
//@Component //這個加上會報錯了，因爲在HandlerProcessor中已經注入到ioc容器了，
//再次加上註解，Service裝配的時候會再次初始化而注入失敗
public class HandlerContext {
private Map<String, Class> handlerMap;
public HandlerContext(Map<String, Class> handlerMap) {
this.handlerMap = handlerMap;
}
public AbstractHandler getInstance(String type) {
Class clazz = handlerMap.get(type);
if (clazz == null) {
throw new IllegalArgumentException("not found handler for type: " + type);
}
return (AbstractHandler) BeanTool.getBean(clazz);
}
}
@Component
public class BeanTool implements ApplicationContextAware {
private static ApplicationContext applicationContext;
//自動初始化注入applicationContext
@Override
public void setApplicationContext(ApplicationContext context) throws BeansException {
if (applicationContext==null){
applicationContext=context;
}
}
/**通過bean的縮寫的方式獲取類型
*
* @param name
* @return
*/
public static Object getBean(String name){
return applicationContext.getBean(name);
}
public static <T> T getBean(Class<T> clazz){
return applicationContext.getBean(clazz);
}
}
-
總結
策略模式的作用：就是把具體的算法實現從業務邏輯中剝離出來，成爲一系列獨立算法類，使得它們可以相互替換。
在我們可以通過繼續寫心的業務邏輯類，來拓展我們的代碼，這個是非常方便的，基本serviceImpl的類中是不用修改的。可以增加新的東西，不用修改舊的東西，這個正好是設計模式裏面的開閉原則。