Setting an exception cause in Java

Question

I can see on catching an exception that I can print e.getCause(), though it is always null.

Do I need to set it somewhere, or is something missing which is setting the cause to null?

Answer 1

An Exception has the attributes message and cause. The message is a description, telling a human reader more or less exactly, what went wrong. The cause is something different: it is, if available, another (nested) Throwable.

The concept is often used if we use custom exceptions like this:

catch(IOException e) {   throw new ApplicationException("Failed on reading file soandso", e);   //                              ^ Message                        ^ Cause } 

In response to djangofan's comment:

The standard is that the nested expression (the cause) is printed with its stack trace too.

Running this little application

public class Exceptions {     public static void main(String[] args) {         Exception r = new RuntimeException("Some message");         throw new RuntimeException("Some other message", r);     } } 

will output

Exception in thread "main" java.lang.RuntimeException: Some other message     at Exceptions.main(Exceptions.java:4)     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) Caused by: java.lang.RuntimeException: Some message     at Exceptions.main(Exceptions.java:3)     ... 5 more 

Both messages are included.

Answer 2

The cause is usually set in the constructor of the exception. Look at public Exception(String message, Throwable cause).

If it isn't set in the constructor, you can call initCause().