没有ServletException的Serlvet的init方法 [英] init method of a Serlvet without ServletException

问题描述

虽然ServletException是一个被检查的异常，我可以通过扩展HttpServlet并创建一个init方法来创建一个Servlet，并抛出或捕获ServletException。编译器没有抱怨，Servlet也工作。

这是如何工作的。

解决方案

覆盖规则之一是：

mC的签名是mA签名的子签名（§8.4.2）。

其中 subsignature 被定义为

两个方法或构造函数M和N具有相同的签名，如果它们的
具有相同的名称，相同的类型参数（如果有的话）（§8.4.4），而
在适应正式参数类型N的类型
参数M，相同的形式参数类型。

方法m1的签名是签名的子签名a
方法m2如果有：

• m2有与m1相同的签名，或




• m1的签名与m2签名的擦除（§4.6）相同。

  / li>
  

所以一个 throws 子句可以忽略在小孩班。但是，给定 m2 覆盖 m1

• 如果m2有一个引用任何检查的异常类型的throws子句，那么
  则m1必须有一个throws子句，或者发生编译时错误。




• 对于m2中throws子句中列出的每个被检查的异常类型，
  同样的异常类或其超类型必须出现在
  擦除（§4.6 ）的m1的throws子句;否则，会出现编译时
  错误。

例如

  class Bar {
 public void method（）throws Exception {} 
} 
 
 class Foo extends Bar {
 @Override 
 public void method（）{} 
} 
  

如果你执行

  Bar bar = new Foo（）; 
 bar.method（）; //你需要处理它，但它永远不会发生
 
 Foo foo = new Foo（）; 
 foo.method（）; //你不需要处理，它仍然不会发生
  

但是， / p>

  class Bar {
 public void method（）throws SomeCheckedException {} 
} 
 
 class Foo extends Bar {
 @Override 
 public void method（）throws SomeOtherUnrelatedCheckedException {} 
} 
  

将失败

  Bar bar = new Foo（）; 
 bar.method（）; // Bar说一些异常可能会发生，但实际的对象可能会抛出另一个异常，它不会被处理。 
  

Though ServletException is a checked exception, I am able create a Servlet by extending HttpServlet and creating a init method with out throwing or catching the ServletException. The compiler didn't complain and the Servlet works as well.

How come this works.

解决方案

One of the rules for overriding is that

The signature of mC is a subsignature (§8.4.2) of the signature of mA.

where subsignature is defined as

Two methods or constructors, M and N, have the same signature if they have the same name, the same type parameters (if any) (§8.4.4), and, after adapting the formal parameter types of N to the the type parameters of M, the same formal parameter types.

The signature of a method m1 is a subsignature of the signature of a method m2 if either:

• m2 has the same signature as m1, or

• the signature of m1 is the same as the erasure (§4.6) of the signature of m2.

So a throws clause can be ignored in the child class. However, given m2 overrides m1

• If m2 has a throws clause that mentions any checked exception types, then m1 must have a throws clause, or a compile-time error occurs.

• For every checked exception type listed in the throws clause of m2, that same exception class or one of its supertypes must occur in the erasure (§4.6) of the throws clause of m1; otherwise, a compile-time error occurs.

Take the example

class Bar {
   public void method() throws Exception {}
}

class Foo extends Bar {
    @Override  
    public void method() {}
}

If you do

Bar bar = new Foo();
bar.method(); // you'll need to handle it, but it will never happen

Foo foo = new Foo();
foo.method(); // you don't need to handle, it still won't ever happen

But the inverse

class Bar {
   public void method() throws SomeCheckedException{}
}

class Foo extends Bar {
    @Override  
    public void method() throws SomeOtherUnrelatedCheckedException{}
}

would fail

Bar bar = new Foo();
bar.method(); // Bar says some exception can happen, but the actual object may throw another, it wouldn't be handled.

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