java 空指针异常java.lang.NullPointerException

绝地灬酷狼 2021-09-27 11:04 515阅读 0赞

Java 空指针异常（java.lang.NullPointerException），顾名思义，即对象没有进行实例化便进行了使用。实例化的意义，就是将对象实例的地址赋值给对象符号。

比如 String a = new String();即在堆中将String的实例构造后，将地址赋值给a。 String a = “123”；即将常量池中的“123”这个对象实例的地址赋值给a。

引发空指针异常，往往是在获取对象实例地址的时候，由于获取的对象实例并不存在，因此返回的地址为null，而又没进行判断，直接使用了该对象实例（地址）引发的。

例如：

String userName = request.getParameter("username");
    if (userName.equals("root"))
    　　{....}

如果request中并不存在“username”字段，则自然getParameter返回的是null。而调用userName（为null）的equals会抛出异常。

小技巧：如果要把某个方法的返回值与常量做比较，把常量放在前面，可以避免调用null对象的equals方法。例如上述改成：

"root".equals(userName);

因此，我们在进行对象实例地址的获取赋值的时候，在之后，要进行必要的判断。

--------------------

When you declare a reference variable (i.e. an object) you are really creating a pointer to an object. Consider the following code where you declare a variable of primitive type `int`:

int x;
    x = 10;

In this example, the variable x is an `int` and Java will initialize it to 0 for you. When you assign it to 10 in the second line your value 10 is written into the memory location pointed to by x.

But, when you try to declare a reference type something different happens. Take the following code:

Integer num;
    num = new Integer(10);

The first line declares a variable named `num`, but, it does not contain a primitive value. Instead, it contains a pointer (because the type is `Integer` which is a reference type). Since you did not say as yet what to point to Java sets it to null, meaning "**I am pointing at nothing**".

In the second line, the `new` keyword is used to instantiate (or create) an object of type Integer and the pointer variable `num` is assigned this object. You can now reference the object using the dereferencing operator `.` (a dot).

The `Exception` that you asked about occurs when you declare a variable but did not create an object. If you attempt to dereference `num` BEFORE creating the object you get a `NullPointerException`. In the most trivial cases, the compiler will catch the problem and let you know that "num may not have been initialized" but sometimes you write code that does not directly create the object.

For instance, you may have a method as follows:

public void doSomething(SomeObject obj) {
       //do something to obj
    }

In which case you are not creating the object `obj`, rather assuming that it was created before the `doSomething` method was called. Unfortunately, it is possible to call the method like this:

doSomething(null);

In which case `obj` is null. If the method is intended to do something to the passed-in object, it is appropriate to throw the `NullPointerException` because it's a programmer error and the programmer will need that information for debugging purposes.

Alternatively, there may be cases where the purpose of the method is not solely to operate on the passed in object, and therefore a null parameter may be acceptable. In this case, you would need to check for a **null parameter** and behave differently. You should also explain this in the documentation. For example, `doSomething` could be written as:

/**
      * @param obj An optional foo for ____. May be null, in which case 
      *  the result will be ____.
      */
    public void doSomething(SomeObject obj) {
        if(obj != null) {
           //do something
        } else {
           //do something else
        }
    }

Finally, [How to pinpoint the exception & cause using Stack Trace][How to pinpoint the exception _ cause using Stack Trace]

参考：

[What is a NullPointerException, and how do I fix it?][What is a NullPointerException_ and how do I fix it]

[Java 空指针异常（java.lang.NullPointerException）][Java _java.lang.NullPointerException]

[How to pinpoint the exception _ cause using Stack Trace]: https://stackoverflow.com/q/3988788/2775450
[What is a NullPointerException_ and how do I fix it]: https://stackoverflow.com/questions/218384/what-is-a-nullpointerexception-and-how-do-i-fix-it
[Java _java.lang.NullPointerException]: https://blog.csdn.net/IterZebra/article/details/6756391