chap10.htm

java书籍《thinking in java》

        at Rethrowing.f(Rethrowing.java:8)
        at Rethrowing.g(Rethrowing.java:12)
        at Rethrowing.main(Rethrowing.java:24)
Caught in main, e.printStackTrace()
java.lang.Exception: thrown from f()
        at Rethrowing.g(Rethrowing.java:18)
        at Rethrowing.main(Rethrowing.java:24)</PRE></FONT></BLOCKQUOTE>
<DIV ALIGN="LEFT"><P><FONT FACE="Georgia">Because of
<B>fillInStackTrace(&#160;)</B>, line 18 becomes the new
<A NAME="Index1088"></A>point of origin of the exception.

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<DIV ALIGN="LEFT"><P><FONT FACE="Georgia">The class <B>Throwable</B> must appear in
the exception specification for <B>g(&#160;)</B> and <B>main(&#160;)</B> because
<B>fillInStackTrace(&#160;)</B> produces a reference to a <B>Throwable</B>
object. Since <A NAME="Index1089"></A><B>Throwable</B> is a base class of
<B>Exception</B>, it&#8217;s possible to get an object that&#8217;s a
<B>Throwable</B> but <I>not</I> an <B>Exception</B>, so the handler for
<B>Exception</B> in <B>main(&#160;) </B>might miss it. To make sure everything
is in order, the compiler forces an exception specification for
<B>Throwable</B>. For example, the exception in the following program is
<I>not</I> caught in <B>main(&#160;)</B>:

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<BLOCKQUOTE><FONT SIZE = "+1"><PRE><font color=#009900>//: c10:ThrowOut.java</font>
<font color=#0000ff>public</font> <font color=#0000ff>class</font> ThrowOut {
  <font color=#0000ff>public</font> <font color=#0000ff>static</font> <font color=#0000ff>void</font>
  main(String[] args) <font color=#0000ff>throws</font> Throwable {
    <font color=#0000ff>try</font> {
      <font color=#0000ff>throw</font> <font color=#0000ff>new</font> Throwable(); 
    } <font color=#0000ff>catch</font>(Exception e) {
      System.err.println(<font color=#004488>"Caught in main()"</font>);
    }
  }
} <font color=#009900>///:~</font></PRE></FONT></BLOCKQUOTE>
<DIV ALIGN="LEFT"><P><FONT FACE="Georgia">It&#8217;s also possible to rethrow a
different exception from the one you caught. If you do this, you get a similar
effect as when you use <B>fillInStackTrace(&#160;)</B>&#8212;the information
about the original site of the exception is lost, and what you&#8217;re left
with is the information pertaining to the new <B>throw</B>:

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<BLOCKQUOTE><FONT SIZE = "+1"><PRE><font color=#009900>//: c10:RethrowNew.java</font>
<font color=#009900>// Rethrow a different object </font>
<font color=#009900>// from the one that was caught.</font>

<font color=#0000ff>class</font> OneException <font color=#0000ff>extends</font> Exception {
  <font color=#0000ff>public</font> OneException(String s) { <font color=#0000ff>super</font>(s); }
}

<font color=#0000ff>class</font> TwoException <font color=#0000ff>extends</font> Exception {
  <font color=#0000ff>public</font> TwoException(String s) { <font color=#0000ff>super</font>(s); }
}

<font color=#0000ff>public</font> <font color=#0000ff>class</font> RethrowNew {
  <font color=#0000ff>public</font> <font color=#0000ff>static</font> <font color=#0000ff>void</font> f() <font color=#0000ff>throws</font> OneException {
    System.out.println(
      <font color=#004488>"originating the exception in f()"</font>);
    <font color=#0000ff>throw</font> <font color=#0000ff>new</font> OneException(<font color=#004488>"thrown from f()"</font>);
  }
  <font color=#0000ff>public</font> <font color=#0000ff>static</font> <font color=#0000ff>void</font> main(String[] args) 
  <font color=#0000ff>throws</font> TwoException {
    <font color=#0000ff>try</font> {
      f();
    } <font color=#0000ff>catch</font>(OneException e) {
      System.err.println(
        <font color=#004488>"Caught in main, e.printStackTrace()"</font>);
      e.printStackTrace(System.err);
      <font color=#0000ff>throw</font> <font color=#0000ff>new</font> TwoException(<font color=#004488>"from main()"</font>);
    }
  }
} <font color=#009900>///:~</font></PRE></FONT></BLOCKQUOTE>
<DIV ALIGN="LEFT"><P><FONT FACE="Georgia">The output is:</FONT><BR></P></DIV>

<BLOCKQUOTE><FONT SIZE = "+1"><PRE>originating the exception in f()
Caught in main, e.printStackTrace()
OneException: thrown from f()
        at RethrowNew.f(RethrowNew.java:17)
        at RethrowNew.main(RethrowNew.java:22)
Exception in thread <font color=#004488>"main"</font> TwoException: from main()
        at RethrowNew.main(RethrowNew.java:27)</PRE></FONT></BLOCKQUOTE>
<DIV ALIGN="LEFT"><P><FONT FACE="Georgia">The final exception knows only that it
came from <B>main(&#160;)</B>, and not from <B>f(&#160;)</B>. 

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<DIV ALIGN="LEFT"><P><FONT FACE="Georgia">You never have to worry about cleaning up
the previous exception, or any exceptions for that matter. They&#8217;re all
heap-based objects created with <B>new</B>, so the garbage collector
automatically cleans them all up.

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</FONT><A NAME="_Toc375545370"></A><A NAME="_Toc481064723"></A><BR></P></DIV>
<A NAME="Heading342"></A><FONT FACE = "Verdana"><H2 ALIGN="LEFT">
Standard Java exceptions</H2></FONT>
<DIV ALIGN="LEFT"><P><FONT FACE="Georgia">The Java class <B>Throwable</B> describes
anything that can be thrown as an exception. There are two general types of
<B>Throwable</B> objects (&#8220;types of&#8221; = &#8220;inherited
from&#8221;). <A NAME="Index1090"></A><B>Error</B> represents compile-time and
system errors that you don&#8217;t worry about catching (except in special
cases). <A NAME="Index1091"></A><B>Exception</B> is the basic type that can be
thrown from any of the standard Java library class methods and from your methods
and run-time accidents. So the Java programmer&#8217;s base type of interest is
<B>Exception</B>. 
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<DIV ALIGN="LEFT"><P><FONT FACE="Georgia">The best way to get an overview of the
exceptions is to browse the HTML Java documentation that you can download from
<I>java.sun.com.</I> It&#8217;s worth doing this once just to get a feel for the
various exceptions, but you&#8217;ll soon see that there isn&#8217;t anything
special between one exception and the next except for the name. Also, the number
of exceptions in Java keeps expanding; basically it&#8217;s pointless to print
them in a book. Any new library you get from a third-party vendor will probably
have its own exceptions as well. The important thing to understand is the
concept and what you should do with the exceptions.

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<DIV ALIGN="LEFT"><P><FONT FACE="Georgia">The basic idea is that the name of the
exception represents the problem that occurred, and the exception name is
intended to be relatively self-explanatory. The exceptions are not all defined
in <B>java.lang</B>; some are created to support other libraries such as
<B>util</B>, <B>net,</B> and <B>io</B>, which you can see from their full class
names or what they are inherited from. For example, all I/O exceptions are
inherited from <B>java.io.IOException</B>.

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</FONT><A NAME="_Toc375545372"></A><A NAME="_Toc481064724"></A><BR></P></DIV>
<A NAME="Heading343"></A><FONT FACE = "Verdana"><H3 ALIGN="LEFT">
The special case of RuntimeException</H3></FONT>
<DIV ALIGN="LEFT"><P><FONT FACE="Georgia">The first example in this chapter
was</FONT><BR></P></DIV>

<BLOCKQUOTE><FONT SIZE = "+1"><PRE><font color=#0000ff>if</font>(t == <font color=#0000ff>null</font>)
  <font color=#0000ff>throw</font> <font color=#0000ff>new</font> NullPointerException();</PRE></FONT></BLOCKQUOTE>
<DIV ALIGN="LEFT"><P><FONT FACE="Georgia">It can be a bit horrifying to think that
you must check for <B>null</B> on every reference that is passed into a method
(since you can&#8217;t know if the caller has passed you a valid reference).
Fortunately, you don&#8217;t&#8212;this is part of the standard run-time
checking that Java performs for you, and if any call is made to a <B>null</B>
reference, Java will automatically throw a
<A NAME="Index1092"></A><A NAME="Index1093"></A><B>NullPointerException</B>. So
the above bit of code is always superfluous. 

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<DIV ALIGN="LEFT"><P><FONT FACE="Georgia">There&#8217;s a whole group of exception
types that are in this category. They&#8217;re always thrown automatically by
Java and you don&#8217;t need to include them in your exception specifications.
Conveniently enough, they&#8217;re all grouped together by putting them under a
single base class called <B>RuntimeException</B>, which is a perfect example of
inheritance: it establishes a family of types that have some characteristics and
behaviors in common. Also, you never need to write an exception specification
saying that a method might throw a <B>RuntimeException</B>, since that&#8217;s
just assumed. Because they indicate bugs, you virtually never catch a
<A NAME="Index1094"></A><A NAME="Index1095"></A><B>RuntimeException</B>&#8212;it&#8217;s
dealt with automatically. If you were forced to check for
<B>RuntimeException</B>s your code could get messy. Even though you don&#8217;t
typically catch <B>RuntimeExceptions</B>,<B> </B>in your own packages you might
choose to throw some of the <B>RuntimeException</B>s. 

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<DIV ALIGN="LEFT"><P><FONT FACE="Georgia">What happens when you don&#8217;t catch
such exceptions? Since the compiler doesn&#8217;t enforce exception
specifications for these, it&#8217;s quite plausible that a
<B>RuntimeException</B> could percolate all the way out to your <B>main(&#160;)
</B>method without being caught. To see what happens in this case, try the
following example:</FONT><BR></P></DIV>

<BLOCKQUOTE><FONT SIZE = "+1"><PRE><font color=#009900>//: c10:NeverCaught.java</font>
<font color=#009900>// Ignoring RuntimeExceptions.</font>

<font color=#0000ff>public</font> <font color=#0000ff>class</font> NeverCaught {
  <font color=#0000ff>static</font> <font color=#0000ff>void</font> f() {
    <font color=#0000ff>throw</font> <font color=#0000ff>new</font> RuntimeException(<font color=#004488>"From f()"</font>);
  }
  <font color=#0000ff>static</font> <font color=#0000ff>void</font> g() {
    f();
  }
  <font color=#0000ff>public</font> <font color=#0000ff>static</font> <font color=#0000ff>void</font> main(String[] args) {
    g();
  }
} <font color=#009900>///:~</font></PRE></FONT></BLOCKQUOTE>
<DIV ALIGN="LEFT"><P><FONT FACE="Georgia">You can already see that a
<B>RuntimeException </B>(or anything inherited from it) is a special case, since
the compiler doesn&#8217;t require an exception specification for these types.

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<DIV ALIGN="LEFT"><P><FONT FACE="Georgia">The output is:</FONT><BR></P></DIV>

<BLOCKQUOTE><FONT SIZE = "+1"><PRE>Exception in thread <font color=#004488>"main"</font>
java.lang.RuntimeException: From f()
        at NeverCaught.f(NeverCaught.java:9