tij0098.html

学习java的经典书籍

<font color="#0000ff">public</font> <font color="#0000ff">class</font> Rethrowing {
  <font color="#0000ff">public</font> <font color="#0000ff">static</font> <font color="#0000ff">void</font> f() <font color="#0000ff">throws</font> Exception {
    System.out.println(
      "originating the exception in f()");
    <font color="#0000ff">throw</font> <font color="#0000ff">new</font> Exception("thrown from f()");
  }
  <font color="#0000ff">public</font> <font color="#0000ff">static</font> <font color="#0000ff">void</font> g() <font color="#0000ff">throws</font> Throwable {
    <font color="#0000ff">try</font> {
      f();
    } <font color="#0000ff">catch</font>(Exception e) {
      System.out.println(
        "Inside g(), e.printStackTrace()");
      e.printStackTrace();
      <font color="#0000ff">throw</font> e; <font color="#009900">// 17</font>
      <font color="#009900">// throw e.fillInStackTrace(); // 18</font>
    }
  }
  <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> {
      g();
    } <font color="#0000ff">catch</font>(Exception e) {
      System.out.println(
        "Caught in main, e.printStackTrace()");
      e.printStackTrace();
    }
  }
} <font color="#009900">///:~ </PRE></font></font><DIV ALIGN=LEFT><P></DIV><DIV ALIGN=LEFT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black">The
important line numbers are marked inside of comments. With line 17 un-commented
(as shown), the output is:
</FONT><P></DIV>

<font color="#990000"><PRE>originating the exception in f()
Inside g(), e.printStackTrace()
java.lang.Exception: thrown from f()
        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.f(Rethrowing.java:8)
        at Rethrowing.g(Rethrowing.java:12)
        at Rethrowing.main(Rethrowing.java:24) </PRE></font><DIV ALIGN=LEFT><P></DIV><DIV ALIGN=LEFT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black">So
the exception stack trace always remembers its true point of origin, no matter
how many times it gets rethrown.
</FONT><P></DIV><DIV ALIGN=LEFT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black">With
line 17 commented and line 18 un-commented, 
</FONT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black"><B>fillInStackTrace(&#160;)</B></FONT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black">
is used instead, and the result is:
</FONT><P></DIV>

<font color="#990000"><PRE>originating the exception in f()
Inside g(), e.printStackTrace()
java.lang.Exception: thrown from f()
        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><DIV ALIGN=LEFT><P></DIV><DIV ALIGN=LEFT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black">Because
of 
</FONT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black"><B>fillInStackTrace(&#160;)</B></FONT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black">,
line 18 becomes the new <A NAME="Index938"></A>point
of origin of the exception.
</FONT><P></DIV><DIV ALIGN=LEFT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black">The
class 
</FONT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black"><B>Throwable</B></FONT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black">
must appear in the exception specification for 
</FONT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black"><B>g(&#160;)</B></FONT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black">
and 
</FONT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black"><B>main(&#160;)</B></FONT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black">
because 
</FONT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black"><B>fillInStackTrace(&#160;)</B></FONT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black">
produces a handle to a 
</FONT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black"><B>Throwable</B></FONT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black">
object. Since <A NAME="Index939"></A></FONT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black"><B>Throwable</B></FONT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black">
is a base class of 
</FONT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black"><B>Exception</B></FONT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black">,
it&#8217;s possible to get an object that&#8217;s a 
</FONT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black"><B>Throwable</B></FONT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black">
but 
</FONT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black"><I>not</I></FONT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black">
an 
</FONT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black"><B>Exception</B></FONT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black">,
so the handler for 
</FONT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black"><B>Exception</B></FONT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black">
in 
</FONT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black"><B>main(&#160;)
</B></FONT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black">might
miss it. To make sure everything is in order, the compiler forces an exception
specification for 
</FONT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black"><B>Throwable</B></FONT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black">.
For example, the exception in the following program is 
</FONT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black"><I>not</I></FONT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black">
caught in 
</FONT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black"><B>main(&#160;)</B></FONT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black">:</FONT><P></DIV>

<font color="#990000"><PRE><font color="#009900">//: 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.out.println("Caught in main()");
    }
  }
} <font color="#009900">///:~ </PRE></font></font><DIV ALIGN=LEFT><P></DIV><DIV ALIGN=LEFT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black">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 
</FONT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black"><B>fillInStackTrace(&#160;)</B></FONT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black">:
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 
</FONT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black"><B>throw</B></FONT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black">:</FONT><P></DIV>

<font color="#990000"><PRE><font color="#009900">//: RethrowNew.java</font>
<font color="#009900">// Rethrow a different object from the one that</font>
<font color="#009900">// was caught</font>

<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> Exception {
    System.out.println(
      "originating the exception in f()");
    <font color="#0000ff">throw</font> <font color="#0000ff">new</font> Exception("thrown from f()");
  }
  <font color="#0000ff">public</font> <font color="#0000ff">static</font> <font color="#0000ff">void</font> main(String[] args) {
    <font color="#0000ff">try</font> {
      f();
    } <font color="#0000ff">catch</font>(Exception e) {
      System.out.println(
        "Caught in main, e.printStackTrace()");
      e.printStackTrace();
      <font color="#0000ff">throw</font> <font color="#0000ff">new</font> NullPointerException("from main");
    }
  }
} <font color="#009900">///:~ </PRE></font></font><DIV ALIGN=LEFT><P></DIV><DIV ALIGN=LEFT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black">The
output is:
</FONT><P></DIV>

<font color="#990000"><PRE>originating the exception in f()
Caught in main, e.printStackTrace()
java.lang.Exception: thrown from f()
        at RethrowNew.f(RethrowNew.java:8)
        at RethrowNew.main(RethrowNew.java:13)
java.lang.NullPointerException: from main
        at RethrowNew.main(RethrowNew.java:18) </PRE></font><DIV ALIGN=LEFT><P></DIV><DIV ALIGN=LEFT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black">The
final exception knows only that it came from 
</FONT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black"><B>main(&#160;)</B></FONT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black">,
and not from 
</FONT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black"><B>f(&#160;)</B></FONT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black">.
Note that 
</FONT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black"><B>Throwable</B></FONT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black">
isn&#8217;t necessary in any of the exception specifications.
</FONT><P></DIV><DIV ALIGN=LEFT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black">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 
</FONT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black"><B>new</B></FONT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black">,
so the garbage collector automatically cleans them all up.
</FONT><a name="_Toc375545370"></a><a name="_Toc408018597"></a><P></DIV>
<HR><DIV ALIGN=LEFT><A NAME="fn42" HREF="#fnB42">[42]</A><FONT FACE="Carmina Md BT" SIZE=2 COLOR="Black">
This is a significant improvement over C++ exception handling, which
doesn&#8217;t catch violations of exception specifications until run time, when
it&#8217;s not very useful.
</FONT><P></DIV>


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