Several cases of java.lang.nullpointerexception error reporting:

1. String variable is not initialized

2. Interface type objects are not initialized with specific classes, such as:

　　List stuList ； In this case, a null pointer exception will be reported

　　List stuList = new ArrayList()； After initialization, no error will be reported

3. When the value of an object is null and you do not judge it as null, a null pointer exception will also be reported

Therefore, add a judgment before the code, such as:

　　　　if(ObjId != null)；

For string type objects, you can make the following judgment:

　　　　if（objectStr ！== null && amp; ！“”. equals（objectStr））

Of course, you can also judge whether it is not an empty string:

　　　　if（objectStr ！== null && amp; ！“”. equals（objectStr.trim()））

Example: when I edit a page, it will pop up a custom error.jsp page code block (reporting system error)

Check the log: (an error is reported on line 54 — the picture is badly cut)

Because doc.getid does not make judgment, null pointer exception is reported when ID does not exist… (for case 3)