Module java.base
Package java.lang

Class Throwable

java.lang.Object
java.lang.Throwable
All Implemented Interfaces:
Serializable
Direct Known Subclasses:
Error,Exception
public classThrowableextendsObjectimplementsSerializable
TheThrowable 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 Javathrow statement. Similarly, only this class or one of its subclasses can be the argument type in acatch clause. For the purposes of compile-time checking of exceptions, Throwable and any subclass ofThrowable that is not also a subclass of eitherRuntimeException orError are regarded as checked exceptions.

Instances of two subclasses,Error andException, 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. Over time, a throwable cansuppress other throwables from being propagated. Finally, the throwable can also contain acause: another throwable that caused this throwable to be constructed. The recording of this causal information is referred to as thechained 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 theCollection interface, and that its persistence is implemented atopjava.io. Suppose the internals of theadd method can throw anIOException. The implementation can communicate the details of theIOException to its caller while conforming to theCollection interface by wrapping theIOException 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 theinitCause(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 theThrowable constructors that takes a cause. Because theinitCause 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 toThrowable.

By convention, classThrowable and its subclasses have two constructors, one that takes no arguments and one that takes aString 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 aThrowable (the cause), and one that takes aString (the detail message) and aThrowable (the cause).

SeeJava Language Specification:
11.2 Compile-Time Checking of Exceptions
Since:
1.0
See Also: