7-b147
Throwable class is the superclass of all errors and
exceptions in the Java language. Only objects that are instances of this
class (or one of its subclasses) are thrown by the Java Virtual Machine or
can be thrown by the Java throw statement. Similarly, only
this class or one of its subclasses can be the argument type in a
catch clause.
For the purposes of compile-time checking of exceptions, Throwable and any subclass of Throwable that is not also a
subclass of either RuntimeExceptionErrorInstances of two subclasses, ErrorException
A throwable contains a snapshot of the execution stack of its thread at the time it was created. It can also contain a message string that gives more information about the error. Over time, a throwable can suppress other throwables from being propagated. Finally, the throwable can also contain a cause: another throwable that caused this throwable to be constructed. The recording of this causal information is referred to as the chained exception facility, as the cause can, itself, have a cause, and so on, leading to a "chain" of exceptions, each caused by another.
One reason that a throwable may have a cause is that the class that throws it is built atop a lower layered abstraction, and an operation on the upper layer fails due to a failure in the lower layer. It would be bad design to let the throwable thrown by the lower layer propagate outward, as it is generally unrelated to the abstraction provided by the upper layer. Further, doing so would tie the API of the upper layer to the details of its implementation, assuming the lower layer's exception was a checked exception. Throwing a "wrapped exception" (i.e., an exception containing a cause) allows the upper layer to communicate the details of the failure to its caller without incurring either of these shortcomings. It preserves the flexibility to change the implementation of the upper layer without changing its API (in particular, the set of exceptions thrown by its methods).
A second reason that a throwable may have a cause is that the method
that throws it must conform to a general-purpose interface that does not
permit the method to throw the cause directly. For example, suppose
a persistent collection conforms to the Collection interface, and that its persistence is implemented atop
java.io. Suppose the internals of the add method
can throw an IOException. The implementation
can communicate the details of the IOException to its caller
while conforming to the Collection interface by wrapping the
IOException in an appropriate unchecked exception. (The
specification for the persistent collection should indicate that it is
capable of throwing such exceptions.)
A cause can be associated with a throwable in two ways: via a
constructor that takes the cause as an argument, or via the
initCause(java.lang.Throwable)Throwable constructors that takes a cause.
Because the initCause method is public, it allows a cause to be
associated with any throwable, even a "legacy throwable" whose
implementation predates the addition of the exception chaining mechanism to
Throwable.
By convention, class Throwable and its subclasses have two
constructors, one that takes no arguments and one that takes a
String argument that can be used to produce a detail message.
Further, those subclasses that might likely have a cause associated with
them should have two more constructors, one that takes a
Throwable (the cause), and one that takes a
String (the detail message) and a Throwable (the
cause).
getStackTrace()null value of this field indicates subsequent calls to setStackTrace(java.lang.StackTraceElement[])fillInStackTrace()getSuppressed()suppressedExceptions field points to a
zero-element list, the field is reset to the sentinel value.
null as its detail message.
The cause is not initialized, and may subsequently be initialized by a
call to initCause(java.lang.Throwable)The fillInStackTrace()
initCause(java.lang.Throwable)The fillInStackTrace()
message the detail message. The detail message is saved for
later retrieval by the getMessage()Note that the detail message associated with
cause is not automatically incorporated in
this throwable's detail message.
The fillInStackTrace()
message the detail message (which is saved for later retrieval
by the getMessage()cause the cause (which is saved for later retrieval by the
getCause()null value is
permitted, and indicates that the cause is nonexistent or
unknown.)(cause==null ? null : cause.toString()) (which
typically contains the class and detail message of cause).
This constructor is useful for throwables that are little more than
wrappers for other throwables (for example, java.security.PrivilegedActionExceptionThe fillInStackTrace()
cause the cause (which is saved for later retrieval by the
getCause()null value is
permitted, and indicates that the cause is nonexistent or
unknown.)getSuppressed()addSuppressed(java.lang.Throwable)fillInStackTrace()null will be written to the
stackTrace field, and subsequent calls to fillInStackTrace and setStackTrace(java.lang.StackTraceElement[])getStackTrace()Note that the other constructors of Throwable treat
suppression as being enabled and the stack trace as being
writable. Subclasses of Throwable should document any
conditions under which suppression is disabled and document
conditions under which the stack trace is not writable.
Disabling of suppression should only occur in exceptional
circumstances where special requirements exist, such as a
virtual machine reusing exception objects under low-memory
situations. Circumstances where a given exception object is
repeatedly caught and rethrown, such as to implement control
flow between two sub-systems, is another situation where
immutable throwable objects would be appropriate.
message the detail message.cause the cause. (A null value is permitted,
and indicates that the cause is nonexistent or unknown.)enableSuppression whether or not suppression is enabled or disabledwritableStackTrace whether or not the stack trace should be
writableOutOfMemoryErrorNullPointerExceptionArithmeticExceptiongetMessage().
null if the
cause is nonexistent or unknown. (The cause is the throwable that
caused this throwable to get thrown.)
This implementation returns the cause that was supplied via one of
the constructors requiring a Throwable, or that was set after
creation with the initCause(java.lang.Throwable)Throwable. Note that it is not
necessary to override any of the PrintStackTrace methods,
all of which invoke the getCause method to determine the
cause of a throwable.
null if the
cause is nonexistent or unknown.This method can be called at most once. It is generally called from
within the constructor, or immediately after creating the
throwable. If this throwable was created
with ThrowableThrowable
An example of using this method on a legacy throwable type without other support for setting the cause is:
try {
lowLevelOp();
} catch (LowLevelException le) {
throw (HighLevelException)
new HighLevelException().initCause(le); // Legacy constructor
}
cause the cause (which is saved for later retrieval by the
getCause()null value is
permitted, and indicates that the cause is nonexistent or
unknown.)Throwable instance.IllegalArgumentException if cause is this
throwable. (A throwable cannot be its own cause.)IllegalStateException if this throwable was
created with Throwable(java.lang.Throwable) Throwable(java.lang.String,java.lang.Throwable) getLocalizedMessage()getLocalizedMessage returns null, then just
the class name is returned.
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 toString()fillInStackTrace()
java.lang.NullPointerException
at MyClass.mash(MyClass.java:9)
at MyClass.crunch(MyClass.java:6)
at MyClass.main(MyClass.java:3)
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
Throwable object information about the current state of
the stack frames for the current thread.
If the stack trace of this Throwable Throwable, calling this method has no effect.
Throwable instance.printStackTrace()printStackTrace()Some virtual machines may, under some circumstances, omit one
or more stack frames from the stack trace. In the extreme case,
a virtual machine that has no stack trace information concerning
this throwable is permitted to return a zero-length array from this
method. Generally speaking, the array returned by this method will
contain one element for every frame that would be printed by
printStackTrace. Writes to the returned array do not
affect future calls to this method.
getStackTrace()printStackTrace()fillInStackTrace()If the stack trace of this Throwable Throwable, calling this method has no effect other than
validating its argument.
stackTrace the stack trace elements to be associated with
this Throwable. The specified array is copied by this
call; changes in the specified array after the method invocation
returns will have no affect on this Throwable's stack
trace.NullPointerException if stackTrace is
null or if any of the elements of
stackTrace are nullindex index of the element to return.IndexOutOfBoundsException if index < 0 ||
index >= getStackTraceDepth() Throwable from a stream, enforcing
well-formedness constraints on fields. Null entries and
self-pointers are not allowed in the list of suppressedExceptions. Null entries are not allowed for stack
trace elements. A null stack trace in the serial form results
in a zero-length stack element array. A single-element stack
trace whose entry is equal to new StackTraceElement("",
"", null, Integer.MIN_VALUE) results in a null stackTrace field.
Note that there are no constraints on the value the cause field can hold; both null and this are
valid values for the field.
try-with-resources statement.
The suppression behavior is enabled unless disabled via a constructor. When suppression is disabled, this method does nothing other than to validate its argument.
Note that when one exception initCause(java.lang.Throwable) another exception, the first
exception is usually caught and then the second exception is
thrown in response. In other words, there is a causal
connection between the two exceptions.
In contrast, there are situations where two independent
exceptions can be thrown in sibling code blocks, in particular
in the try block of a try-with-resources
statement and the compiler-generated finally block
which closes the resource.
In these situations, only one of the thrown exceptions can be
propagated. In the try-with-resources statement, when
there are two such exceptions, the exception originating from
the try block is propagated and the exception from the
finally block is added to the list of exceptions
suppressed by the exception from the try block. As an
exception unwinds the stack, it can accumulate multiple
suppressed exceptions.
An exception may have suppressed exceptions while also being caused by another exception. Whether or not an exception has a cause is semantically known at the time of its creation, unlike whether or not an exception will suppress other exceptions which is typically only determined after an exception is thrown.
Note that programmer written code is also able to take advantage of calling this method in situations where there are multiple sibling exceptions and only one can be propagated.
exception the exception to be added to the list of
suppressed exceptionsIllegalArgumentException if exception is this
throwable; a throwable cannot suppress itself.NullPointerException if exception is nulltry-with-resources
statement, in order to deliver this exception.
If no exceptions were suppressed or Throwable(java.lang.String,java.lang.Throwable,boolean,boolean) , an empty array is returned. This method is
thread-safe. Writes to the returned array do not affect future
calls to this method.