Throwable Error Exception RuntimeException 解析

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, java.lang.Error and 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. Over time, a throwable can plain Throwable#addSuppressed suppress other throwables from being propagated.  Finally, the throwable can also contain a <i>cause</i>: another throwable that caused this throwable to be constructed.  The recording of this causal information is referred to as the <i>chained exception</i> 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  java.util.Collection Collection interface, and that its persistence is implemented atop java.io.  Suppose the internals of the  add method can throw an  java.io.IOException 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).

Error 

An Error is a subclass of Throwable that indicates serious problems that a reasonable application should not try to catch. Most such errors are abnormal conditions.
The ThreadDeath error, though a "normal" condition,is also a subclass of Error because most applications should not try to catch it.
A method is not required to declare in its throws clause any subclasses of Error that might be thrown during the execution of the method but not caught, since these errors are abnormal conditions that should never occur.
That is,  Error and its subclasses are regarded as unchecked exceptions for the purposes of compile-time checking of exceptions.

Exception

The class Exception and its subclasses are a form of{@code Throwable} that indicates conditions that a reasonable application might want to catch.

The class Exception and any subclasses that are not also subclasses of RuntimeException are checked exceptions. Checked exceptions need to be declared in a method or constructor's throws clause if they can be thrown by the execution of the method or constructor and propagate outside the method or constructor boundary.

RuntimeException 

RuntimeException is the superclass of those exceptions that can be thrown during the normal operation of the Java Virtual Machine.

RuntimeException and its subclasses are unchecked  exceptions.  Unchecked exceptions do not need to be declared in a method or constructor's  throws clause if they  can be thrown by the execution of the method or constructor and propagate outside the method or constructor boundary.

Throwable
|-Error
----|-LinkageError
--------|-NoClassDefFoundError
--------|-IncompatibleClassChangeError
------------|-NoSuchMethodError
--------|-ThreadDeath
|-Exception
----|-TimeoutException
----|-RuntimeException
--------|-IndexOutOfBoundsException
--------|-NetworkOnMainThreadException
--------|-FilesException
--------|-NullPointerException
--------|-ActivityNotFoundException
--------|-IllegalArgumentException
------------|-NumberFormatException
------------|-InvalidPathException
------------|-UnsupportedCharsetException
------------|-InvalidParameterException
------------|-IllegalFormatException
------------|-ArithmeticException
------------|-IllegalStateException