* Instances of two subclasses, {@link java.lang.Error} and {@link java.lang.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 {@link java.util.Collection Collection} * interface, and that its persistence is implemented atop {@code java.io}. Suppose the internals of * the {@code add} method can throw an {@link java.io.IOException IOException}. The implementation * can communicate the details of the {@code IOException} to its caller while conforming to the * {@code Collection} interface by wrapping the {@code 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 {@link #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 {@code Throwable} constructors that takes a cause. * * Because the {@code 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 {@code Throwable}. * *
* By convention, class {@code Throwable} and its subclasses have two constructors, one that takes * no arguments and one that takes a {@code 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 {@code Throwable} (the cause), and one that takes a * {@code String} (the detail message) and a {@code Throwable} (the cause). * */ public class Throwable implements Serializable { /** * Constructs a new throwable with {@code null} as its detail message. The cause is not initialized, * and may subsequently be initialized by a call to {@link #initCause}. * *
* The {@link #fillInStackTrace()} method is called to initialize the stack trace data in the newly * created throwable. */ public Throwable() { throw new RuntimeException(); } /** * Constructs a new throwable with the specified detail message. The cause is not initialized, and * may subsequently be initialized by a call to {@link #initCause}. * *
* The {@link #fillInStackTrace()} method is called to initialize the stack trace data in the newly * created throwable. * * @param message * the detail message. The detail message is saved for later retrieval by the * {@link #getMessage()} method. */ public Throwable(String message) { throw new RuntimeException(); } /** * Constructs a new throwable with the specified detail message and cause. *
* Note that the detail message associated with {@code cause} is not automatically * incorporated in this throwable's detail message. * *
* The {@link #fillInStackTrace()} method is called to initialize the stack trace data in the newly * created throwable. * * @param message * the detail message (which is saved for later retrieval by the {@link #getMessage()} * method). * @param cause * the cause (which is saved for later retrieval by the {@link #getCause()} method). (A * {@code null} value is permitted, and indicates that the cause is nonexistent or unknown.) */ public Throwable(String message, @Nullable Throwable cause) { throw new RuntimeException(); } /** * Constructs a new throwable with the specified cause and a detail message of * {@code (cause==null ? null : cause.toString())} (which typically contains the class and detail * message of {@code cause}). This constructor is useful for throwables that are little more than * wrappers for other throwables. * *
* The {@link #fillInStackTrace()} method is called to initialize the stack trace data in the newly * created throwable. * * @param cause * the cause (which is saved for later retrieval by the {@link #getCause()} method). (A * {@code null} value is permitted, and indicates that the cause is nonexistent or unknown.) */ public Throwable(@Nullable Throwable cause) { throw new RuntimeException(); } /** * Appends the specified exception to the exceptions that were suppressed in order to deliver this * exception. This method is thread-safe and typically called (automatically and implicitly) by the * {@code try}-with-resources statement. * *
* Note that when one exception {@linkplain #initCause(Throwable) 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 {@code try} block of a {@code try}-with-resources statement and the * compiler-generated {@code finally} block which closes the resource. * * In these situations, only one of the thrown exceptions can be propagated. In the {@code try} * -with-resources statement, when there are two such exceptions, the exception originating from the * {@code try} block is propagated and the exception from the {@code finally} block is added to the * list of exceptions suppressed by the exception from the {@code 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. * * @param exception * the exception to be added to the list of suppressed exceptions * @throws IllegalArgumentException * if {@code exception} is this throwable; a throwable cannot suppress itself. * @throws NullPointerException * if {@code exception} is {@code null} */ public final void addSuppressed(@Nullable Throwable exception) { throw new RuntimeException(); } /** * Fills in the execution stack trace. This method records within this {@code Throwable} object * information about the current state of the stack frames for the current thread. * * @return a reference to this {@code Throwable} instance. * @see java.lang.Throwable#printStackTrace() */ public Throwable fillInStackTrace() { throw new RuntimeException(); } /** * Returns the cause of this throwable or {@code 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 * {@code Throwable}, or that was set after creation with the {@link #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 {@code Throwable}. Note that it is not necessary to * override any of the {@code PrintStackTrace} methods, all of which invoke the {@code getCause} * method to determine the cause of a throwable. * * @return the cause of this throwable or {@code null} if the cause is nonexistent or unknown. */ @Nullable public Throwable getCause() { throw new RuntimeException(); } /** * Creates a localized description of this throwable. Subclasses may override this method in order * to produce a locale-specific message. For subclasses that do not override this method, the * default implementation returns the same result as {@code getMessage()}. * * @return The localized description of this throwable. */ @Nullable public String getLocalizedMessage() { throw new RuntimeException(); } /** * Returns the detail message string of this throwable. * * @return the detail message string of this {@code Throwable} instance (which may be {@code null}). */ @Nullable public String getMessage() { throw new RuntimeException(); } /** * Provides programmatic access to the stack trace information printed by * {@link #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 * {@code printStackTrace}. Writes to the returned array do not affect future calls to this method. * * @return an array of stack trace elements representing the stack trace pertaining to this * throwable. */ public StackTraceElement[] getStackTrace() { throw new RuntimeException(); } /** * Returns an array containing all of the exceptions that were suppressed, typically by the * {@code 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. * * @return an array containing all of the exceptions that were suppressed to deliver this exception. */ public final Throwable[] getSuppressed() { throw new RuntimeException(); } /** * Initializes the cause of this throwable to the specified value. (The cause is the * throwable that caused this throwable to get thrown.) * *
* 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 * {@link #Throwable(Throwable)} or {@link #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
* }
*
*
* @param cause
* the cause (which is saved for later retrieval by the {@link #getCause()} method). (A
* {@code null} value is permitted, and indicates that the cause is nonexistent or unknown.)
* @return a reference to this {@code Throwable} instance.
* @throws IllegalArgumentException
* if {@code cause} is this throwable. (A throwable cannot be its own cause.)
* @throws IllegalStateException
* if this throwable was created with {@link #Throwable(Throwable)} or
* {@link #Throwable(String,Throwable)}, or this method has already been called on this
* throwable.
*/
public Throwable initCause(@Nullable Throwable cause) {
throw new RuntimeException();
}
/**
* Prints this throwable and its backtrace to the standard error stream. This method prints a stack
* trace for this {@code Throwable} object on the error output stream that is the value of the field
* {@code System.err}. The first line of output contains the result of the {@link #toString()}
* method for this object. Remaining lines represent data previously recorded by the method
* {@link #fillInStackTrace()}. The format of this information depends on the implementation, but
* the following example may be regarded as typical: * *This example was produced by running the program: * ** 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 {@code "..."}. 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 {@code 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() { throw new RuntimeException(); } /** * Prints this throwable and its backtrace to the specified print stream. * * @param s * {@code PrintStream} to use for output */ public void printStackTrace(PrintStream s) { throw new RuntimeException(); } /** * Returns a short description of this throwable. The result is the concatenation of: *