java.lang.Throwable
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java.lang.Object
public class Throwable
The 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 RuntimeException or Error are
regarded as checked exceptions.
Instances of two subclasses, Error and
Exception, are conventionally used to indicate
that exceptional situations have occurred. Typically, these instances
are freshly created in the context of the exceptional situation so
as to include relevant information (such as stack trace data).
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. Finally, it can contain a cause: another throwable that caused this throwable to get thrown. The cause facility is new in release 1.4. It is also known 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(Throwable) method. New throwable classes that
wish to allow causes to be associated with them should provide constructors
that take a cause and delegate (perhaps indirectly) to one of the
Throwable constructors that takes a cause. For example:
try {
lowLevelOp();
} catch (LowLevelException le) {
throw new HighLevelException(le); // Chaining-aware constructor
}
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. For example:
try {
lowLevelOp();
} catch (LowLevelException le) {
throw (HighLevelException)
new HighLevelException().initCause(le); // Legacy constructor
}
Prior to release 1.4, there were many throwables that had their own
non-standard exception chaining mechanisms (
ExceptionInInitializerError, ClassNotFoundException,
UndeclaredThrowableException,
InvocationTargetException,
WriteAbortedException,
PrivilegedActionException,
PrinterIOException,
RemoteException and
NamingException).
All of these throwables have been retrofitted to
use the standard exception chaining mechanism, while continuing to
implement their "legacy" chaining mechanisms for compatibility.
Further, as of release 1.4, many general purpose Throwable
classes (for example Exception, RuntimeException,
Error) have been retrofitted with constructors that take
a cause. This was not strictly necessary, due to the existence of the
initCause method, but it is more convenient and expressive to
delegate to a constructor that takes a cause.
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).
Also introduced in release 1.4 is the getStackTrace() method,
which allows programmatic access to the stack trace information that was
previously available only in text form, via the various forms of the
printStackTrace() method. This information has been added to the
serialized representation of this class so getStackTrace
and printStackTrace will operate properly on a throwable that
was obtained by deserialization.
| Constructor Summary | |
|---|---|
Throwable()
Constructs a new throwable with null as its detail message. |
|
Throwable(String message)
Constructs a new throwable with the specified detail message. |
|
Throwable(String message,
Throwable cause)
Constructs a new throwable with the specified detail message and cause. |
|
Throwable(Throwable cause)
Constructs a new throwable with the specified cause and a detail message of (cause==null ? null : cause.toString()) (which typically contains the class and detail message of cause). |
|
| Method Summary | |
|---|---|
Throwable |
fillInStackTrace()
Suppressed. Fills in the execution stack trace. |
Throwable |
getCause()
Returns the cause of this throwable or null if the
cause is nonexistent or unknown. |
String |
getLocalizedMessage()
Suppressed. Creates a localized description of this throwable. |
String |
getMessage()
Suppressed. Returns the detail message string of this throwable. |
StackTraceElement[] |
getStackTrace()
Suppressed. Provides programmatic access to the stack trace information printed by printStackTrace(). |
Throwable |
initCause(Throwable cause)
Suppressed. Initializes the cause of this throwable to the specified value. |
void |
printStackTrace()
Suppressed. Prints this throwable and its backtrace to the standard error stream. |
void |
printStackTrace(PrintStream s)
Suppressed. Prints this throwable and its backtrace to the specified print stream. |
void |
printStackTrace(PrintWriter s)
Suppressed. Prints this throwable and its backtrace to the specified print writer. |
void |
setStackTrace(StackTraceElement[] stackTrace)
Suppressed. Sets the stack trace elements that will be returned by getStackTrace() and printed by printStackTrace()
and related methods. |
String |
toString()
Suppressed. Returns a short description of this throwable. |
| Methods inherited from class java.lang.Object |
|---|
clone, equals, finalize, getClass, hashCode, notify, notifyAll, wait, wait, wait |
| Constructor Detail |
|---|
public Throwable()
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() method is called to initialize
the stack trace data in the newly created throwable.
public Throwable(String message)
initCause(java.lang.Throwable).
The fillInStackTrace() method is called to initialize
the stack trace data in the newly created throwable.
message - the detail message. The detail message is saved for
later retrieval by the getMessage() method.
public Throwable(String message,
Throwable cause)
Note that the detail message associated with
cause is not automatically incorporated in
this throwable's detail message.
The fillInStackTrace() method is called to initialize
the stack trace data in the newly created throwable.
message - the detail message (which is saved for later retrieval
by the getMessage() method).cause - the cause (which is saved for later retrieval by the
getCause() method). (A null value is
permitted, and indicates that the cause is nonexistent or
unknown.)public Throwable(Throwable cause)
PrivilegedActionException).
The fillInStackTrace() method is called to initialize
the stack trace data in the newly created throwable.
cause - the cause (which is saved for later retrieval by the
getCause() method). (A null value is
permitted, and indicates that the cause is nonexistent or
unknown.)| Method Detail |
|---|
public String getMessage()
public String getLocalizedMessage()
getMessage().
public Throwable getCause()
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(Throwable) method. While it is
typically unnecessary to override this method, a subclass can override
it to return a cause set by some other means. This is appropriate for
a "legacy chained throwable" that predates the addition of chained
exceptions to 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.public Throwable initCause(Throwable cause)
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 Throwable(Throwable) or
Throwable(String,Throwable), this method cannot be called
even once.
cause - the cause (which is saved for later retrieval by the
getCause() method). (A 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(Throwable) or
Throwable(String,Throwable), or this method has already
been called on this throwable.public String toString()
getLocalizedMessage()
method
toString in class Objectpublic void printStackTrace()
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() method for
this object. Remaining lines represent data previously recorded by
the method 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)
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 {
}
public void printStackTrace(PrintStream s)
s - PrintStream to use for outputpublic void printStackTrace(PrintWriter s)
s - PrintWriter to use for outputpublic Throwable fillInStackTrace()
Throwable object information about the current state of
the stack frames for the current thread.
Throwable instance.printStackTrace()public StackTraceElement[] getStackTrace()
printStackTrace(). Returns an array of stack trace elements,
each representing one stack frame. The zeroth element of the array
(assuming the array's length is non-zero) represents the top of the
stack, which is the last method invocation in the sequence. Typically,
this is the point at which this throwable was created and thrown.
The last element of the array (assuming the array's length is non-zero)
represents the bottom of the stack, which is the first method invocation
in the sequence.
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.
public void setStackTrace(StackTraceElement[] stackTrace)
getStackTrace() and printed by printStackTrace()
and related methods.
This method, which is designed for use by RPC frameworks and other
advanced systems, allows the client to override the default
stack trace that is either generated by fillInStackTrace()
when a throwable is constructed or deserialized when a throwable is
read from a serialization stream.
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 null
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