Prints this throwable and its backtrace to the standard error stream. This method prints a stack
trace for this
Throwable object on the error output stream that is the value of the field
System.err. The first line of output contains the result of the
Throwable.toString()
method for this object. Remaining lines represent data previously recorded by the method
Throwable.fillInStackTrace(). The format of this information depends on the implementation, but
the following example may be regarded as typical:
java.lang.NullPointerException
at MyClass.mash(MyClass.java:9)
at MyClass.crunch(MyClass.java:6)
at MyClass.main(MyClass.java:3)
This example was produced by running the program:
class MyClass {
public static void main(String[] args) {
crunch(null);
}
static void crunch(int[] a) {
mash(a);
}
static void mash(int[] b) {
System.out.println(b[0]);
}
}
The backtrace for a throwable with an initialized, non-null cause should generally include the
backtrace for the cause. The format of this information depends on the implementation, but the
following example may be regarded as typical:
HighLevelException: MidLevelException: LowLevelException
at Junk.a(Junk.java:13)
at Junk.main(Junk.java:4)
Caused by: MidLevelException: LowLevelException
at Junk.c(Junk.java:23)
at Junk.b(Junk.java:17)
at Junk.a(Junk.java:11)
... 1 more
Caused by: LowLevelException
at Junk.e(Junk.java:30)
at Junk.d(Junk.java:27)
at Junk.c(Junk.java:21)
... 3 more
Note the presence of lines containing the characters
"...". These lines indicate that the
remainder of the stack trace for this exception matches the indicated number of frames from the
bottom of the stack trace of the exception that was caused by this exception (the "enclosing"
exception). This shorthand can greatly reduce the length of the output in the common case where a
wrapped exception is thrown from same method as the "causative exception" is caught. The above
example was produced by running the program:
public class Junk {
public static void main(String[] args) {
try {
a();
} catch (HighLevelException e) {
e.printStackTrace();
}
}
static void a() throws HighLevelException {
try {
b();
} catch (MidLevelException e) {
throw new HighLevelException(e);
}
}
static void b() throws MidLevelException {
c();
}
static void c() throws MidLevelException {
try {
d();
} catch (LowLevelException e) {
throw new MidLevelException(e);
}
}
static void d() throws LowLevelException {
e();
}
static void e() throws LowLevelException {
throw new LowLevelException();
}
}
class HighLevelException extends Exception {
HighLevelException(Throwable cause) {
super(cause);
}
}
class MidLevelException extends Exception {
MidLevelException(Throwable cause) {
super(cause);
}
}
class LowLevelException extends Exception {
}
As of release 7, the platform supports the notion of
suppressed exceptions (in conjunction
with the
try-with-resources statement). Any exceptions that were suppressed in order to
deliver an exception are printed out beneath the stack trace. The format of this information
depends on the implementation, but the following example may be regarded as typical:
Exception in thread "main" java.lang.Exception: Something happened
at Foo.bar(Foo.java:10)
at Foo.main(Foo.java:5)
Suppressed: Resource$CloseFailException: Resource ID = 0
at Resource.close(Resource.java:26)
at Foo.bar(Foo.java:9)
... 1 more
Note that the "... n more" notation is used on suppressed exceptions just at it is used on
causes. Unlike causes, suppressed exceptions are indented beyond their "containing exceptions."
An exception can have both a cause and one or more suppressed exceptions:
Exception in thread "main" java.lang.Exception: Main block
at Foo3.main(Foo3.java:7)
Suppressed: Resource$CloseFailException: Resource ID = 2
at Resource.close(Resource.java:26)
at Foo3.main(Foo3.java:5)
Suppressed: Resource$CloseFailException: Resource ID = 1
at Resource.close(Resource.java:26)
at Foo3.main(Foo3.java:5)
Caused by: java.lang.Exception: I did it
at Foo3.main(Foo3.java:8)
Likewise, a suppressed exception can have a cause:
Exception in thread "main" java.lang.Exception: Main block
at Foo4.main(Foo4.java:6)
Suppressed: Resource2$CloseFailException: Resource ID = 1
at Resource2.close(Resource2.java:20)
at Foo4.main(Foo4.java:5)
Caused by: java.lang.Exception: Rats, you caught me
at Resource2$CloseFailException.<init>(Resource2.java:45)
... 2 more