java.lang.NullPointerException is one of the most popular exceptions in java programming. Anybody working in java must have seen this popping out of nowhere in the java standalone program as well as the java web application.
NullPointerException is a runtime exception, so we don’t need to catch it in the program. NullPointerException is raised in an application when we are trying to do some operation on null where an object is required. Some of the common reasons for NullPointerException in java programs are:
Let’s look at some examples of NullPointerException in java programs.
public class Temp {
public static void main(String[] args) {
Temp t = initT();
t.foo("Hi");
}
private static Temp initT() {
return null;
}
public void foo(String s) {
System.out.println(s.toLowerCase());
}
}
When we run the above program, it throws the following NullPointerException error message.
Exception in thread "main" java.lang.NullPointerException
at Temp.main(Temp.java:7)
We are getting NullPointerException in the statement t.foo("Hi");
because “t” is null here.
public class Temp {
public int x = 10;
public static void main(String[] args) {
Temp t = initT();
int i = t.x;
}
private static Temp initT() {
return null;
}
}
The above program throws the following NullPointerException stack trace.
Exception in thread "main" java.lang.NullPointerException
at Temp.main(Temp.java:9)
NullPointerException is being thrown in statement int i = t.x;
because “t” is null here.
public class Temp {
public static void main(String[] args) {
foo(null);
}
public static void foo(String s) {
System.out.println(s.toLowerCase());
}
}
This is one of the most common occurrences of java.lang.NullPointerException
because it’s the caller who is passing the null argument.
The below image shows the null pointer exception when the above program is executed in Eclipse IDE.
public class Temp {
public static void main(String[] args) {
throw null;
}
}
Below is the exception stack trace of the above program, showing NullPointerException because of throw null;
statement.
Exception in thread "main" java.lang.NullPointerException
at Temp.main(Temp.java:5)
public class Temp {
public static void main(String[] args) {
int[] data = null;
int len = data.length;
}
}
Exception in thread "main" java.lang.NullPointerException
at Temp.main(Temp.java:7)
public class Temp {
public static void main(String[] args) {
int[] data = null;
int len = data[2];
}
}
Exception in thread "main" java.lang.NullPointerException
at Temp.main(Temp.java:7)
public class Temp {
public static String mutex = null;
public static void main(String[] args) {
synchronized(mutex) {
System.out.println("synchronized block");
}
}
}
The synchronized(mutex)
will throw NullPointerException because the “mutex” object is null.
Sometimes we get an error page response from a java web application with an error message as “HTTP Status 500 – Internal Server Error” and root cause as java.lang.NullPointerException
.
For this, I edited the Spring MVC Example project and changed the HomeController method as below.
@RequestMapping(value = "/user", method = RequestMethod.POST)
public String user(@Validated User user, Model model) {
System.out.println("User Page Requested");
System.out.println("User Name: "+user.getUserName().toLowerCase());
System.out.println("User ID: "+user.getUserId().toLowerCase());
model.addAttribute("userName", user.getUserName());
return "user";
}
The below image shows the error message thrown by the web application response.
Here is the exception stack trace:
HTTP Status 500 – Internal Server Error
Type Exception Report
Message Request processing failed; nested exception is java.lang.NullPointerException
Description The server encountered an unexpected condition that prevented it from fulfilling the request.
Exception
org.springframework.web.util.NestedServletException: Request processing failed; nested exception is java.lang.NullPointerException
org.springframework.web.servlet.FrameworkServlet.processRequest(FrameworkServlet.java:982)
org.springframework.web.servlet.FrameworkServlet.doPost(FrameworkServlet.java:872)
javax.servlet.http.HttpServlet.service(HttpServlet.java:661)
org.springframework.web.servlet.FrameworkServlet.service(FrameworkServlet.java:846)
javax.servlet.http.HttpServlet.service(HttpServlet.java:742)
org.apache.tomcat.websocket.server.WsFilter.doFilter(WsFilter.java:52)
Root Cause
java.lang.NullPointerException
com.journaldev.spring.controller.HomeController.user(HomeController.java:38)
sun.reflect.NativeMethodAccessorImpl.invoke0(Native Method)
sun.reflect.NativeMethodAccessorImpl.invoke(NativeMethodAccessorImpl.java:62)
sun.reflect.DelegatingMethodAccessorImpl.invoke(DelegatingMethodAccessorImpl.java:43)
java.lang.reflect.Method.invoke(Method.java:498)
The root cause is NullPointerException in the statement user.getUserId().toLowerCase()
because user.getUserId()
is returning null.
Detecting NullPointerException is very easy, just look at the exception trace and it will show you the class name and line number of the exception. Then look at the code and see what could be null causing the NullPointerException. Just look at all the above examples, it’s very clear from stack trace what is causing null pointer exception.
java.lang.NullPointerException is an unchecked exception, so we don’t have to catch it. The null pointer exceptions can be prevented using null checks and preventive coding techniques. Look at below code examples showing how to avoid java.lang.NullPointerException
.
if(mutex ==null) mutex =""; //preventive coding
synchronized(mutex) {
System.out.println("synchronized block");
}
//using null checks
if(user!=null && user.getUserName() !=null) {
System.out.println("User Name: "+user.getUserName().toLowerCase());
}
if(user!=null && user.getUserName() !=null) {
System.out.println("User ID: "+user.getUserId().toLowerCase());
}
1. Let’s consider the below function and look out for scenario causing null pointer exception.
public void foo(String s) {
if(s.equals("Test")) {
System.out.println("test");
}
}
The NullPointerException can occur if the argument is being passed as null. The same method can be written as below to avoid NullPointerException.
public void foo(String s) {
if ("Test".equals(s)) {
System.out.println("test");
}
}
2. We can also add null check for argument and throw IllegalArgumentException
if required.
public int getArrayLength(Object[] array) {
if(array == null) throw new IllegalArgumentException("array is null");
return array.length;
}
3. We can use ternary operator as shown in the below example code.
String msg = (str == null) ? "" : str.substring(0, str.length()-1);
4. Use String.valueOf()
rather than toString()
method. For example check PrintStream println() method code is defined as below.
public void println(Object x) {
String s = String.valueOf(x);
synchronized (this) {
print(s);
newLine();
}
}
The below code snippet shows the example where the valueOf() method is used instead of toString().
Object mutex = null;
//prints null
System.out.println(String.valueOf(mutex));
//will throw java.lang.NullPointerException
System.out.println(mutex.toString());
5. Write methods returning empty objects rather than null wherever possible, for example, empty list, empty string, etc.
6. There are some methods defined in collection classes to avoid NullPointerException, you should use them. For example contains(), containsKey(), and containsValue().
Reference: API Document
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Pls Resolve these two errors I got totally stuck and frustrated for two days and not get any solution pls help me to solve my issue, Error is as follow pls… ************* Error No =1:-************ HTTP Status 500 – Internal Server Error Type Exception Report Message null Description The server encountered an unexpected condition that prevented it from fulfilling the request. Exception java.lang.NumberFormatException: null java.lang.Integer.parseInt(Unknown Source) java.lang.Integer.parseInt(Unknown Source) com.ServletAdd.AddServlet.service(AddServlet.java:13) javax.servlet.http.HttpServlet.service(HttpServlet.java:741) org.apache.tomcat.websocket.server.WsFilter.doFilter(WsFilter.java:53) Note The full stack trace of the root cause is available in the server logs ******** Error No= 2:-**************** HTTP Status 404 – Not Found Type Status Report Message /ServletInterfaceDemo/NewFile.html Description The origin server did not find a current representation for the target resource or is not willing to disclose that one exists.
- Nafees Kausar
Causes bad lag on joining minecraft 1.4.2 servers. Error executing task on Client java.lang.NullPointerException at csx.d(SourceFile:199) at dhk.a(SourceFile:1990) at my.a(SourceFile:122) at my.a(SourceFile:16) at kc.a(SourceFile:15) at agk.h(SourceFile:135) at ago.h(SourceFile:23) at agk.p(SourceFile:114) at agk.bf(SourceFile:99) at cvk.e(SourceFile:915) at cvk.b(SourceFile:411) at net.minecraft.client.main.Main.main(SourceFile:154)
- joe
thank’s it really helps !!
- errami