What is 500 error in jsp

The jsp interface I wrote yesterday encountered such a problem. A 500 error occurred every time it was accessed. It was an abnormal error in the java code. After debugging for a long time, I didn't find any problem with the code, and I felt very desperate.

The error message is as follows:

The code is as follows:

<div>
     list = (ArrayList<jsonobject>)request.getAttribute("newsList");
        for(JSONObject j: list){

    %>
    <a>"></a>
    <p>...</p>

    
</jsonobject>
</div>

According to the error message, I found that the problem lies in this line of code:

ArrayList<jsonobject> list = (ArrayList<jsonobject>)request.getAttribute("newsList");</jsonobject></jsonobject>

Intuitively, there is no error, and the parameter name in request.getAttribute("newsList") is not wrong either.
The corresponding servlet also reads the contents of the database.
Why is a 500 error reported?

The code is modified as follows:

<div>
     list = (ArrayList<jsonobject>)request.getAttribute("newsList");
        if (list != null && list.size()!=0){
            for(JSONObject j: list){

    %>
    <a>"></a>
    <p>...</p>

    
</jsonobject>
</div>

That is, non-empty judgment is made between directly traversing the printing list! ! !
This also provides me with great inspiration for writing code in the future.
There is still a long way to go in terms of code specifications, coding habits and code rigor! ! !

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