EventActionDispatcher 可以发布“this”吗？在构造函数完成之前？

发布于 2024-07-15 19:41:50 字数 880 浏览 4 评论 0原文

使用 EventActionDispatcher 的推荐方法如下（根据 API 文档 @ http://struts.apache.org/1.2.9/api/org/apache/struts/actions/EventActionDispatcher.html ）

   public class MyCustomAction extends Action {

       protected ActionDispatcher dispatcher = new EventActionDispatcher(this);

       public ActionForward execute(ActionMapping mapping,
                                    ActionForm form,
                                    HttpServletRequest request,
                                    HttpServletResponse response)
                           throws Exception {
           return dispatcher.execute(mapping, form, request, response);
       }
   }

这样做是否会在构造函数退出？管理方法之外的字段分配的规则是什么。

提前致谢。

真挚地，莱斯

原文

The recommended way to use the EventActionDispatcher is as follows (per the API docs @ http://struts.apache.org/1.2.9/api/org/apache/struts/actions/EventActionDispatcher.html )

   public class MyCustomAction extends Action {

       protected ActionDispatcher dispatcher = new EventActionDispatcher(this);

       public ActionForward execute(ActionMapping mapping,
                                    ActionForm form,
                                    HttpServletRequest request,
                                    HttpServletResponse response)
                           throws Exception {
           return dispatcher.execute(mapping, form, request, response);
       }
   }

Does doing this publish the reference to "this" before the constructor exits? What are the rules governing field assignments outside of methods.

Thanks in advance.

Sincerely,
LES

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旧情别恋 2024-07-22 19:41:50

每周需要 3 个人追踪一次……如果 EventActionDispatcher 启动一个线程或对线程执行任何导致使用“this”的操作，“this”可能为 null。

永远不要在构造函数完成之前传递“this”，否则在线程情况下您将面临“this”为 null 的风险。

我所做的是将“init()”方法添加到需要执行类似操作的类中，并在创建对象后调用它。

还有其他微妙之处，例如这个例子：

public abstract class Foo
{
    protected Foo()
    {
        car();
    }

    public abstract void car();
}

public class Bar 
    extends Foo
{
    private final String value;

    public Bar(final String str)
    {
        value = str;
    }

    public void car()
    {
        // this line will crash because value is null
        System.out.println(value.charAt(0));
    }
}

public class Main
{
    public static void main(final String[] argv)
    {
        final Foo foo;

        foo = new Bar("Hello");
    }
}

最安全的做法是：

在构造函数返回之前永远不要使用“this”
永远不要从构造函数中调用任何类自己的非静态方法，除非该类是最终类。

您可以调用最终方法，但您必须确保它们不会调用可重写的方法...这可能意味着事情会崩溃...所以不这样做更安全。

That took 3 people a week to track down once... "this" can be null if the EventActionDispatcher starts a thread or does anything with a thread that causes the "this" to be used.

NEVER pass "this" before the constructor has completed or you run the risk of "this" being null in the case of threading.

What I do is add an "init()" method to my classes that need to do things like that and call it after I create the object.

There are also other subtleties, such as this example:

public abstract class Foo
{
    protected Foo()
    {
        car();
    }

    public abstract void car();
}

public class Bar 
    extends Foo
{
    private final String value;

    public Bar(final String str)
    {
        value = str;
    }

    public void car()
    {
        // this line will crash because value is null
        System.out.println(value.charAt(0));
    }
}

public class Main
{
    public static void main(final String[] argv)
    {
        final Foo foo;

        foo = new Bar("Hello");
    }
}

The safest thing to do is:

never use "this" before the constructor has returned
never call any of the classes own non-static methods from a constructor unless the class is final.

You can can call final methods, but you have to be sure that they do not call overrideable methods... and that can mean things break down the road... so safer not to do it.

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