CDI(Weld)高级<2> Interceptors(拦截器)

1.拦截器综述

拦截器的功能是定义在Java拦截器规范。

拦截器规范定义了三种拦截点:

1. 业务方法拦截,
2. 生命周期回调侦听,
3. 超时拦截(EJB)方法。

在容器的生命周期中进行拦截

public class DependencyInjectionInterceptor {
   @PostConstruct 
   public void injectDependencies(InvocationContext ctx) { ... }
}

EJB超时时使用的拦截器

public class TimeoutInterceptor {

   @AroundTimeout 
   public Object manageTransaction(InvocationContext ctx) throws Exception { ... }
}

在业务上,对某一个Bean的方法进行拦截

public class TransactionInterceptor {

   @AroundInvoke 
   public Object manageTransaction(InvocationContext ctx) throws Exception { ... }
}

@AroundInvoke注释指定了要用做拦截器的方法，拦截器方法与被拦截的业务方法执行同一个java调用堆栈、同一个事务和安全上下文中。用@AroundInvoke注释指定的方法必须遵照如下格式：public Object XXX(javax.interceptor.InvocationContext ctx) throws Exception

下面是javax.interceptor.InvocationContext封装了客户端所调用业务方法的一些信息。

package javax.interceptor;
public interface InvocationContext{
    public Object getTarget();
    public Method getMethod();
    public Ojbect[] getParameters();
    public void setParameters(Object[] newArgs);
    public java.util.Map<String, Ojbect> getContextData();
    public Object proceed() throws Exception;
}

• getTarget() 指向被调用的bean实例
• getMethod() 指向被拦截的业务方法
• getParameters() 获取被拦截业务方法的参数
• setParameters() 设置被拦截业务方法的参数
• getContextData() 返回一个Map对象，它在整个方法调用期间均可以被访问到。位于同一个方法调用内的不一样拦截器之间能够利用它来传递上下文相关的数据。

示例:

//被拦截的方法
@Interceptors(HelloInterceptor.class) 
public class HelloChinaBean {
    public String SayHello(String name) {
        return name +"Hello World.";
    }
}

//拦截器定义
public class HelloInterceptor {
    @AroundInvoke
    public Object log(InvocationContext ctx) throws Exception {
       
        try{
            if (ctx.getMethod().getName().equals("SayHello")){
                System.out.println("Holle World!!!" );
            }            
            return ctx.proceed();
        }catch (Exception e) {
            throw e;
        } 
    }
}

2.拦截器绑定(Interceptor bindings)

假设咱们想要申明一些bean的事务。咱们先要的是一个拦截器绑定类型来指定哪些bean咱们要申明.
首先定义一个注解

@InterceptorBinding
@Target({METHOD, TYPE})
@Retention(RUNTIME)
public @interface Transactional {}

如今咱们能够很容易地指定类ShoppingCart是事务性对象:

@Transactional
public class ShoppingCart { ... }

或者咱们能够指定一个方法的事务

public class ShoppingCart {
   @Transactional public void checkout() { ... }
}

2.拦截器实现(Implementing interceptors)

咱们实际上要实现提供了这种事务管理方面的拦截器,因此咱们须要作的是建立一个标准的拦截,并配上@Interceptor和@transactional注解.

@Transactional @Interceptor
public class TransactionInterceptor {

   @AroundInvoke 
   public Object manageTransaction(InvocationContext ctx) throws Exception { ... }
}

拦截器能够利用依赖注入:

@Transactional @Interceptor
public class TransactionInterceptor {

    @Resource UserTransaction transaction;

    @AroundInvoke 
    public Object manageTransaction(InvocationContext ctx) throws Exception { ... }
}

多个拦截器可使用相同的拦截器绑定类型。

@Resource和@Inject的区别:

Injection Mechanism	Can Inject JNDI Resources Directly	Can Inject Regular Classes Directly	Resolves By	Typesafe
Resource Injection	Yes	No	Resource name	No
Dependency Injection	No	Yes	Type	Yes

3.启用拦截器(Enabling interceptors)

默认状况下,全部拦截器被禁用.要使用拦截器.须要在bean.xml中进行配置,以启用.从CDI 1.1起拦截器可使用@Priority注释为整个应用程序启用。

<beans
   xmlns="http://xmlns.jcp.org/xml/ns/javaee"
   xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance"
   xsi:schemaLocation="
      http://xmlns.jcp.org/xml/ns/javaee
      http://xmlns.jcp.org/xml/ns/javaee/beans_1_1.xsd">
   <interceptors>
      <class>org.mycompany.myapp.TransactionInterceptor</class>
   </interceptors>
</beans>

这样有2个好处:

• 拦截器比较重要,在XML中确保其肯定性行为
• 它让咱们在部署时启用或禁用拦截器类。

固然也能够配置启用多个拦截器

<beans
   xmlns="http://xmlns.jcp.org/xml/ns/javaee"
   xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance"
   xsi:schemaLocation="
      http://xmlns.jcp.org/xml/ns/javaee
      http://xmlns.jcp.org/xml/ns/javaee/beans_1_1.xsd">
   <interceptors>
      <class>org.mycompany.myapp.SecurityInterceptor</class>
      <class>org.mycompany.myapp.TransactionInterceptor</class>
   </interceptors>
</beans>

拦截器毕竟比较重要,不推荐使用@Priority启用.
在CDI中,XML配置的优先级高于@Priority.
关于@Priority能够参考下列:

public static class Interceptor.Priority
extends Object
Priorities that define the order in which interceptors are invoked. These values should be used with the Priority annotation.

Interceptors defined by platform specifications should have priority values in the range PLATFORM_BEFORE up until LIBRARY_BEFORE, or starting at PLATFORM_AFTER.
Interceptors defined by extension libraries should have priority values in the range LIBRARY_BEFORE up until APPLICATION, or LIBRARY_AFTER up until PLATFORM_AFTER.
Interceptors defined by applications should have priority values in the range APPLICATION up until LIBRARY_AFTER.
An interceptor that must be invoked before or after another defined interceptor can choose any appropriate value.

Interceptors with smaller priority values are called first. If more than one interceptor has the same priority, the relative order of these interceptor is undefined.

For example, an extension library might define an interceptor like this:

@Priority(Interceptor.Priority.LIBRARY_BEFORE+10)
 @Interceptor
 public class ValidationInterceptor { ... }

4.Interceptor bindings with members(拦截器注解属性)

假设咱们想要添加一些额外的信息给咱们的@transactional注解:

@InterceptorBinding
@Target({METHOD, TYPE})
@Retention(RUNTIME)
public @interface Transactional {

   boolean requiresNew() default false;
}

CDI将使用requiresNew的值选择两个不一样的拦截器,TransactionInterceptor和RequiresNewTransactionInterceptor

下面是requiresNew为true的拦截器

@Transactional(requiresNew = true) @Interceptor
public class RequiresNewTransactionInterceptor {

   @AroundInvoke 
   public Object manageTransaction(InvocationContext ctx) throws Exception { ... }
}

以下使用:

@Transactional(requiresNew = true)
public class ShoppingCart { ... }

可是若是咱们只有一个拦截器,咱们但愿容器拦截器绑定时忽略requiresNew的值,也许这些信息只用于拦截器实现。咱们可使用@Nonbinding注释:

@InterceptorBinding
@Target({METHOD, TYPE})
@Retention(RUNTIME)
public @interface Secure {

   @Nonbinding String[] rolesAllowed() default {};
}

5.Multiple interceptor binding annotations(多重拦截器绑定注解)

一般咱们使用拦截器绑定的组合类型绑定多个拦截器bean。例如,下面的声明将用于绑定TransactionInterceptor和SecurityInterceptor这2个拦截器到ShoppingCart.

@Secure(rolesAllowed="admin") @Transactional
public class ShoppingCart { ... }

然而,在很是复杂的状况下,一个拦截器自己可能指定拦截器绑定类型:

@Transactional @Secure @Interceptor
public class TransactionalSecureInterceptor { ... }

那么这个拦截器能够绑定到checkout() 方法,如下任何组合均可使用:

public class ShoppingCart {
   @Transactional @Secure public void checkout() { ... }
}

@Secure
public class ShoppingCart {

   @Transactional public void checkout() { ... }
}

@Transactional
public class ShoppingCart {

   @Secure public void checkout() { ... }
}

@Transactional @Secure
public class ShoppingCart {

   public void checkout() { ... }
}

6. Interceptor binding type inheritance(拦截器绑定类型继承)

Java语言支持注解的一个限制就是缺少注解继承.注解应该重用内置已有的.就如同下面这段代码表达的意思

//实际没这写法
public @interface Action extends Transactional, Secure { ... }

幸运的是,CDI围绕Java没有的这个特性开展了一些工做.
咱们会标注一个拦截器绑定类型,其有其余拦截器的绑定类型,(称为元注解)
表述起来有点费劲,就如同下面代码这样.

@Transactional @Secure
@InterceptorBinding
@Target(TYPE)
@Retention(RUNTIME)
public @interface Action { ... }

如今任何Bean绑定 Action这个注解 ,其实就是绑定到了@Transactional @Secure.(就是拦截器TransactionInterceptor和拦截器SecurityInterceptor). (甚至TransactionalSecureInterceptor,若是它存在.)

7.Use of @Interceptors(同时用多个拦截器)

这个注解@Interceptors是拦截器规范定义的,cdi是支持的<使用托管bean和EJB规范>.以下:

@Interceptors({TransactionInterceptor.class, SecurityInterceptor.class})
public class ShoppingCart {

   public void checkout() { ... }
}

但缺点也很明显,不推荐使用.缺点以下:

1. 拦截器在代码中是硬编码.
2. 拦截器在部署时很差更改.
3. 拦截器命令是非全局的——它是在类级别由拦截器的顺序列出.

所以仍是使用上面CDI的使用方式比较好.