public class Throwable extends Object implements Serializable
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, 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(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.
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).
| Constructor and Description |
|---|
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). |
| Modifier and Type | Method and Description |
|---|---|
void |
addSuppressed(Throwable exception)
Appends the specified exception to the exceptions that were suppressed in order to deliver this
exception.
|
Throwable |
fillInStackTrace()
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()
Creates a localized description of this throwable.
|
String |
getMessage()
Returns the detail message string of this throwable.
|
StackTraceElement[] |
getStackTrace()
Provides programmatic access to the stack trace information printed by
printStackTrace(). |
Throwable[] |
getSuppressed()
Returns an array containing all of the exceptions that were suppressed, typically by the
try-with-resources statement, in order to deliver this exception. |
Throwable |
initCause(Throwable cause)
Initializes the cause of this throwable to the specified value.
|
void |
printStackTrace()
Prints this throwable and its backtrace to the standard error stream.
|
void |
printStackTrace(PrintStream s)
Prints this throwable and its backtrace to the specified print stream.
|
String |
toString()
Returns a short description of this throwable.
|
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, @Nullable 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(@Nullable Throwable cause)
(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.
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.)public final void addSuppressed(@Nullable Throwable exception)
try-with-resources statement.
Note that when one exception causes 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 nullpublic Throwable fillInStackTrace()
Throwable object
information about the current state of the stack frames for the current thread.Throwable instance.printStackTrace()@Nullable 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.@Nullable public String getLocalizedMessage()
getMessage().@Nullable public String getMessage()
Throwable instance (which may be null).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. Writes to the returned array do not affect future calls to this method.
public final Throwable[] getSuppressed()
try-with-resources statement, in order to deliver this exception.
If no exceptions were suppressed an empty array is returned. This method is thread-safe. Writes
to the returned array do not affect future calls to this method.public Throwable initCause(@Nullable 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.
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() 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 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)
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
public void printStackTrace(PrintStream s)
s - PrintStream to use for outputpublic String toString()
getLocalizedMessage() method
getLocalizedMessage returns null, then just the class name is returned.