Singleton 單例模式
單例模式是確保每個應用程序只存在一個實例的機制。默認情況下，Spring將所有bean創建爲單例。
你用@Autowired獲取的bean，全局唯一。
@RestController
public class LibraryController {
@Autowired
private BookRepository repository;
@GetMapping("/count")
public Long findCount() {
System.out.println(repository);
return repository.count();
}
}
工廠方法模式
Spring 定義了BeanFactory接口，抽象對象容器：
public interface BeanFactory {
getBean(Class<T> requiredType);
getBean(Class<T> requiredType, Object... args);
getBean(String name);
// ...
]
每一個getBean 方法其實就是一個工廠方法。
代理模式(Proxy)
在Spring中，對於事務，我們可以加一個@Transactional註解，
@Service
public class BookManager {
@Autowired
private BookRepository repository;
@Transactional
public Book create(String author) {
System.out.println(repository.getClass().getName());
return repository.create(author);
}
}
Spring框架，通過AOP做Proxy。
Decorator裝飾器模式
Spring 中的TransactionAwareCacheDecorator 就做了對Cache 的包裝：
public interface Cache {
String getName();
Object getNativeCache();
@Nullable
Cache.ValueWrapper get(Object var1);
@Nullable
<T> T get(Object var1, @Nullable Class<T> var2);
@Nullable
<T> T get(Object var1, Callable<T> var2);
void put(Object var1, @Nullable Object var2);
}
TransactionAwareCacheDecorator 實現了Cache接口，構造時傳入一個targetCache，在調用put等方法時，增加了自己裝飾邏輯在裏面。
public class TransactionAwareCacheDecorator implements Cache {
private final Cache targetCache;
public TransactionAwareCacheDecorator(Cache targetCache) {
Assert.notNull(targetCache, "Target Cache must not be null");
this.targetCache = targetCache;
}
public void put(final Object key, @Nullable final Object value) {
if (TransactionSynchronizationManager.isSynchronizationActive()) {
TransactionSynchronizationManager.registerSynchronization(new TransactionSynchronizationAdapter() {
public void afterCommit() {
TransactionAwareCacheDecorator.this.targetCache.put(key, value);
}
});
} else {
this.targetCache.put(key, value);
}
}
最佳實踐
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裝飾模式是繼承的有力補充。相比於繼承，裝飾模式可以增加代碼的可維護性、擴展性、複用性。在一些情況下裝飾模式可以替代繼承，解決類膨脹問題。
-
裝飾模式有利於程序的可擴展性。在一個項目中，有很多因素考慮不周，特別是業務的變更。通過裝飾模式重新封裝一個裝飾類，可以避免修改繼承體系中的中間類，而是使用裝飾類修飾中間類，這樣原有的程序沒有變更，通過擴展完成了這次變更。
組合模式(Composite)
Spring actuate 提供HealthIndicator, 用於監控服務健康狀態。
@FunctionalInterface
public interface HealthIndicator {
/**
* Return an indication of health.
* @return the health for
*/
Health health();
}
實現類裏，有一個CompositeHealthIndicator, 可以add多個HealthIndicator,放入indicators裏，最後返回health時，聚合所有indicators的Health。
public class CompositeHealthIndicator implements HealthIndicator {
private final Map<String, HealthIndicator> indicators;
private final HealthAggregator healthAggregator;
/**
* Create a new {@link CompositeHealthIndicator}.
* @param healthAggregator the health aggregator
*/
public CompositeHealthIndicator(HealthAggregator healthAggregator) {
this(healthAggregator, new LinkedHashMap<>());
}
/**
* Create a new {@link CompositeHealthIndicator} from the specified indicators.
* @param healthAggregator the health aggregator
* @param indicators a map of {@link HealthIndicator}s with the key being used as an
* indicator name.
*/
public CompositeHealthIndicator(HealthAggregator healthAggregator,
Map<String, HealthIndicator> indicators) {
Assert.notNull(healthAggregator, "HealthAggregator must not be null");
Assert.notNull(indicators, "Indicators must not be null");
this.indicators = new LinkedHashMap<>(indicators);
this.healthAggregator = healthAggregator;
}
public void addHealthIndicator(String name, HealthIndicator indicator) {
this.indicators.put(name, indicator);
}
@Override
public Health health() {
Map<String, Health> healths = new LinkedHashMap<>();
for (Map.Entry<String, HealthIndicator> entry : this.indicators.entrySet()) {
healths.put(entry.getKey(), entry.getValue().health());
}
return this.healthAggregator.aggregate(healths);
}
}
感謝您的認真閱讀。
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