What is the use of printStackTrace() method in Java?

Question

I am going through a socket program. In it, printStackTrace is called on the IOException object in the catch block.
What does printStackTrace() actually do?

catch(IOException ioe)
{
    ioe.printStackTrace();
}

I am unaware of its purpose. What is it used for?

Footpoundsecond

One of the most powerful features in modern IDE's is the ability to look up documentation (called javadoc) for a given Java method. In Eclipse it is Shift-F2 when the cursor is placed on the printStackTrace method name. – Sucre

Sounds like you're new to Java. Here's something to read: today.java.net/article/2006/04/04/… – Bidet

You can also read the code of the printStackTrace() to see exactly what it does and how it does it. – Hillard

What a cool functionality! i dont know it before. But i guess it will be better if i can make the fonts displayed bigger, just like the real web browser.@ThorbjørnRavnAndersen – Isothere

@MarkZar Eclipse by default use an internal browser. You can set it to use the system browser, which use your normal browser with its normal settings. – Sucre

I often see this pattern inside catch blocks: e.printStackTrace(); System.exit(1); Here I want to ask the same question as the OP, but in a different vein: What exactly is the purpose of this? Won't the JRE automatically print the stacktrace and exit, when the exception is thrown? I.e., there isn't really very much "Exception Handling" going on here, right?? – Estivation

It's also generally a very bad idea, as it suppresses the exception - unfortunately it's what Eclipse and other IDEs often auto-suggest as the body for the catch block, so it's used far more often than it should be. – Vere

And now Error Prone will warn you about it :) – Vere

Answer 1

It's a method on Exception instances that prints the stack trace of the instance to System.err.

It's a very simple, but very useful tool for diagnosing an exceptions. It tells you what happened and where in the code this happened.

Here's an example of how it might be used in practice:

try {
    // ...
} catch (SomeException e) { 
    e.printStackTrace();
}

Note that in "serious production code" you usually don't want to do this, for various reasons (such as System.out being less useful and not thread safe). In those cases you usually use some log framework that provides the same (or very similar) output using a command like log.error("Error during frobnication", e);.

Impact

And thanks to checked exceptions it's probably the #1 content of catch blocks :-) – Rasping

@Rasping Or throw new RuntimeException(e) – Moo

however it is considered bad practice as it is not threadsafe. Use the logger statement and include it there. – Lavoie

Answer 2

I was kind of curious about this too, so I just put together a little sample code where you can see what it is doing:

try {
    throw new NullPointerException();
}
catch (NullPointerException e) {
    System.out.println(e);
}

try {
    throw new IOException();
}
catch (IOException e) {
    e.printStackTrace();
}
System.exit(0);

Calling println(e):

java.lang.NullPointerException

Calling e.printStackTrace():

java.io.IOException
      at package.Test.main(Test.java:74)

Creole

This is normal, with the 1st one you call the toString() of the error, and in the 2nd you ask to print all the stackTrace from the error. That's 2 différents things. – Batty

System.err.println(e); will be more appropriate. – Divvy

Answer 3

It helps to trace the exception. For example you are writing some methods in your program and one of your methods causes bug. Then printstack will help you to identify which method causes the bug. Stack will help like this:

First your main method will be called and inserted to stack, then the second method will be called and inserted to the stack in LIFO order and if any error occurs somewhere inside any method then this stack will help to identify that method.

Ritaritardando

Answer 4

printStackTrace() helps the programmer to understand where the actual problem occurred. printStacktrace() is a method of the class Throwable of java.lang package. It prints several lines in the output console. The first line consists of several strings. It contains the name of the Throwable sub-class & the package information. From second line onwards, it describes the error position/line number beginning with at.

The last line always describes the destination affected by the error/exception. The second last line informs us about the next line in the stack where the control goes after getting transfer from the line number described in the last line. The errors/exceptions represents the output in the form a stack, which were fed into the stack by fillInStackTrace() method of Throwable class, which itself fills in the program control transfer details into the execution stack. The lines starting with at, are nothing but the values of the execution stack. In this way the programmer can understand where in code the actual problem is.

Along with the printStackTrace() method, it's a good idea to use e.getmessage().

Kingwood

Answer 5

printStackTrace() prints the locations where the exception occurred in the source code, thus allowing the author who wrote the program to see what went wrong. But since it shows problems in the source code, the user(s) who may or may not have any coding experience may not be able to understand what went wrong, so if the program allows the user to send error messages to the authors, the users may not be able to give good data on what went wrong.

You should consider the Logger.getLogger() method, it offers a better exception handling (logging) facility, and besides printStackTrace() without arguments is considered to be obsolete and should ONLY be used for debugging purposes, not for user display.

Upper

Answer 6

printStackTrace is a method of the Throwable class. This method displays error message in the console; where we are getting the exception in the source code. These methods can be used with catch block and they describe:

1. Name of the exception.
2. Description of the exception.
3. Location of the exception in the source code.

The three methods which describe the exception on the console (in which printStackTrace is one of them) are:

1. printStackTrace()
2. toString()
3. getMessage()

Example:

public class BabluGope {
    public static void main(String[] args) {
        try {
            System.out.println(10/0);
         } catch (ArithmeticException e) {
             e.printStackTrace();   
             // System.err.println(e.toString());
             //System.err.println(e.getMessage());  
         }
    }
}

Ottava

Answer 7

The printStackTrace() helps the programmer understand where the actual problem occurred. The printStackTrace() method is a member of the class Throwable in the java.lang package.

Rao

Answer 8

printStackTrace() helps the programmer to understand where the actual problem occurred. It helps to trace the exception. it is printStackTrace() method of Throwable class inherited by every exception class. This method prints the same message of e object and also the line number where the exception occurred.

The following is an another example of print stack of the Exception in Java.

public class Demo {
   public static void main(String[] args) {
      try {
         ExceptionFunc();
      } catch(Throwable e) {
         e.printStackTrace();
      }
   }
   public static void ExceptionFunc() throws Throwable {
      Throwable t = new Throwable("This is new Exception in Java...");

      StackTraceElement[] trace = new StackTraceElement[] {
         new StackTraceElement("ClassName","methodName","fileName",5)
      };
      t.setStackTrace(trace);
      throw t;
   }
}

java.lang.Throwable: This is new Exception in Java... at ClassName.methodName(fileName:5)

Highbrow

Answer 9

What is the use of e.printStackTrace() method in Java?

Well, the purpose of using this method e.printStackTrace(); is to see what exactly wrong is.

For example, we want to handle an exception. Let's have a look at the following Example.

public class Main{

  public static void main(String[] args) {

    int a = 12;
    int b = 2;

    try {
       int result = a / (b - 2);
       System.out.println(result);
    }

    catch (Exception e)
    {
       System.out.println("Error: " + e.getMessage());
       e.printStackTrace();
    }
  }
}

I've used method e.printStackTrace(); in order to show exactly what is wrong.

In the output, we can see the following result.

Error: / by zero

java.lang.ArithmeticException: / by zero

  at Main.main(Main.java:10)
  at sun.reflect.NativeMethodAccessorImpl.invoke0(Native Method)
  at sun.reflect.NativeMethodAccessorImpl.invoke(NativeMethodAccessorImpl.java:62)
  at sun.reflect.DelegatingMethodAccessorImpl.invoke(DelegatingMethodAccessorImpl.java:43)
  at java.lang.reflect.Method.invoke(Method.java:498)
  at com.intellij.rt.execution.application.AppMain.main(AppMain.java:147)

Transceiver