从Spring框架看设计模式如何灵活使用

Singleton 单例模式

单例模式是确保每个应用程序只存在一个实例的机制。默认情况下,Spring将所有bean创建为单例。

单例模式

你用@Autowired获取的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();
    }
}

工厂方法模式

Spring 定义了BeanFactory接口,抽象对象容器:

public interface BeanFactory {

    getBean(Class<T> requiredType);
    getBean(Class<T> requiredType, Object... args);
    getBean(String name);

    // ...
]

每一个getBean 方法其实就是一个工厂方法。

代理模式(Proxy)

代理模式

在Spring中,对于事务,我们可以加一个@Transactional注解,

@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,通过AOP做Proxy。

Decorator装饰器模式

Spring 中的TransactionAwareCacheDecorator 就做了对Cache 的包装:

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 实现了Cache接口,构造时传入一个targetCache,在调用put等方法时,增加了自己装饰逻辑在里面。


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);
        }

    }

最佳实践

  • 装饰模式是继承的有力补充。相比于继承,装饰模式可以增加代码的可维护性、扩展性、复用性。在一些情况下装饰模式可以替代继承,解决类膨胀问题。

  • 装饰模式有利于程序的可扩展性。在一个项目中,有很多因素考虑不周,特别是业务的变更。通过装饰模式重新封装一个装饰类,可以避免修改继承体系中的中间类,而是使用装饰类修饰中间类,这样原有的程序没有变更,通过扩展完成了这次变更。

组合模式(Composite)

Spring actuate 提供HealthIndicator, 用于监控服务健康状态。

@FunctionalInterface
public interface HealthIndicator {

    /**
     * Return an indication of health.
     * @return the health for
     */
    Health health();

}

实现类里,有一个CompositeHealthIndicator, 可以add多个HealthIndicator,放入indicators里,最后返回health时,聚合所有indicatorsHealth


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);
    }

}

感谢您的认真阅读。

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posted @ 2021-02-08 18:05  JadePeng  阅读(224)  评论(0编辑  收藏  举报