浅述 Spring / SpringMVC 框架中用到的哪些设计模式（上）

// 通过 ConcurrentHashMap（线程安全） 实现单例注册表
private final Map<String, Object> singletonObjects = new ConcurrentHashMap<String, Object>(64);
    public Object getSingleton(String beanName, ObjectFactory<?> singletonFactory) {
        Assert.notNull(beanName, "'beanName' must not be null");
        synchronized (this.singletonObjects) {
            // 检查缓存中是否存在实例  
            Object singletonObject = this.singletonObjects.get(beanName);
            if (singletonObject == null) {
                // 此处省略了很多代码
                try {
                    singletonObject = singletonFactory.getObject();
                }
                // 此处省略了很多代码
                // 如果实例对象在不存在，我们注册到单例注册表中。
                addSingleton(beanName, singletonObject);
            }
            return (singletonObject != NULL_OBJECT ? singletonObject : null);
        }
    }
    //将对象添加到单例注册表
    protected void addSingleton(String beanName, Object singletonObject) {
        synchronized (this.singletonObjects) {
            this.singletonObjects.put(beanName, (singletonObject != null ? singletonObject : NULL_OBJECT));
        }
    }
}
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@FunctionalInterface
public interface HibernateCallback<T> {
   /**
    * Gets called by {@code HibernateTemplate.execute} with an active
    * Hibernate {@code Session}. Does not need to care about activating
    * or closing the {@code Session}, or handling transactions.
    * <p>Allows for returning a result object created within the callback,
    * i.e. a domain object or a collection of domain objects.
    * A thrown custom RuntimeException is treated as an application exception:
    * It gets propagated to the caller of the template.
    *
    * @param session active Hibernate session
    * @return a result object, or {@code null} if none
    * @throws HibernateException if thrown by the Hibernate API
    * @see HibernateTemplate#execute
    */
   @Nullable
   T doInHibernate(Session session) throws HibernateException;
}
public interface HibernateOperations {
    ...
    @Nullable
    <T> T execute(HibernateCallback<T> action) throws DataAccessException;
    ...
}
public class HibernateTemplate implements HibernateOperations, InitializingBean {
    @Override
    @Nullable
    public <T> T execute(HibernateCallback<T> action) throws DataAccessException {
       return doExecute(action, false);
    }
    @Nullable
    protected <T> T doExecute(HibernateCallback<T> action, boolean enforceNativeSession) throws DataAccessException {
       Assert.notNull(action, "Callback object must not be null");
       Session session = null;
       boolean isNew = false;
       ...
       try {
          enableFilters(session);
          Session sessionToExpose = (enforceNativeSession || isExposeNativeSession() ? session : createSessionProxy(session));
          return action.doInHibernate(sessionToExpose);
       }
       ...
       finally {
          if (isNew) {
             SessionFactoryUtils.closeSession(session);
          }
          else {
             disableFilters(session);
          }
       }
    }
    ...
}
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# 抽象角色
public interface DataSource extends CommonDataSource, Wrapper {
    // 尝试与此数据源对象表示的数据源建立连接。
    Connection getConnection() throws SQLException;
    Connection getConnection(String username, String password) throws SQLException;
}
# 装饰类
public class DelegatingDataSource implements DataSource, InitializingBean {
    @Nullable
    private DataSource targetDataSource;
    public DelegatingDataSource() {
    }
    public DelegatingDataSource(DataSource targetDataSource) {
        this.setTargetDataSource(targetDataSource);
    }
    public void setTargetDataSource(@Nullable DataSource targetDataSource) {
        this.targetDataSource = targetDataSource;
    }
    @Nullable
    public DataSource getTargetDataSource() {
        return this.targetDataSource;
    }
    protected DataSource obtainTargetDataSource() {
        DataSource dataSource = this.getTargetDataSource();
        Assert.state(dataSource != null, "No 'targetDataSource' set");
        return dataSource;
    }
    public void afterPropertiesSet() {
        if (this.getTargetDataSource() == null) {
            throw new IllegalArgumentException("Property 'targetDataSource' is required");
        }
    }
    public Connection getConnection() throws SQLException {
        return this.obtainTargetDataSource().getConnection();
    }
    public Connection getConnection(String username, String password) throws SQLException {
        return this.obtainTargetDataSource().getConnection(username, password);
    }
    public PrintWriter getLogWriter() throws SQLException {
        return this.obtainTargetDataSource().getLogWriter();
    }
    public void setLogWriter(PrintWriter out) throws SQLException {
        this.obtainTargetDataSource().setLogWriter(out);
    }
    public int getLoginTimeout() throws SQLException {
        return this.obtainTargetDataSource().getLoginTimeout();
    }
    public void setLoginTimeout(int seconds) throws SQLException {
        this.obtainTargetDataSource().setLoginTimeout(seconds);
    }
    public <T> T unwrap(Class<T> iface) throws SQLException {
        return iface.isInstance(this) ? this : this.obtainTargetDataSource().unwrap(iface);
    }
    public boolean isWrapperFor(Class<?> iface) throws SQLException {
        return iface.isInstance(this) || this.obtainTargetDataSource().isWrapperFor(iface);
    }
    public Logger getParentLogger() {
        return Logger.getLogger("global");
    }
}
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// 装饰类
public class ServerHttpResponseDecorator implements ServerHttpResponse {
   private final ServerHttpResponse delegate;
   public ServerHttpResponseDecorator(ServerHttpResponse delegate) {
      Assert.notNull(delegate, "Delegate is required");
      this.delegate = delegate;
   }
   public ServerHttpResponse getDelegate() {
      return this.delegate;
   }
   // ServerHttpResponse delegation methods...
   @Override
   public boolean setStatusCode(@Nullable HttpStatus status) {
      return getDelegate().setStatusCode(status);
   }
   @Override
   public HttpStatus getStatusCode() {
      return getDelegate().getStatusCode();
   }
   @Override
   public HttpHeaders getHeaders() {
      return getDelegate().getHeaders();
   }
   @Override
   public MultiValueMap<String, ResponseCookie> getCookies() {
      return getDelegate().getCookies();
   }
   @Override
   public void addCookie(ResponseCookie cookie) {
      getDelegate().addCookie(cookie);
   }
   @Override
   public DataBufferFactory bufferFactory() {
      return getDelegate().bufferFactory();
   }
   ...
}
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@FunctionalInterface
public interface ApplicationListener<E extends ApplicationEvent> extends EventListener {
    void onApplicationEvent(E var1);
}
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@FunctionalInterface
public interface ApplicationEventPublisher {
    default void publishEvent(ApplicationEvent event) {
        this.publishEvent((Object)event);
    }
    void publishEvent(Object var1);
}
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