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学习java的经典书籍

</FONT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black"><B>Worm</B></FONT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black">.</FONT><P></DIV><DIV ALIGN=LEFT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black">The
point of all this was to make something reasonably complex that couldn&#8217;t
easily be serialized. The act of serializing, however, is quite simple. Once the 
</FONT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black"><B>ObjectOutputStream</B></FONT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black">
is created from some other stream, 
</FONT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black"><B>writeObject(&#160;)</B></FONT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black">
serializes the object. Notice the call to 
</FONT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black"><B>writeObject(&#160;)</B></FONT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black">
for a 
</FONT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black"><B>String</B></FONT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black">,
as well. You can also write all the primitive data types using the same methods
as 
</FONT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black"><B>DataOutputStream</B></FONT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black">
(they share the same interface).
</FONT><P></DIV><DIV ALIGN=LEFT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black">There
are two separate 
</FONT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black"><B>try</B></FONT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black">
blocks that look similar. The first writes and reads a file and the second, for
variety, writes and reads a 
</FONT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black"><B>ByteArray</B></FONT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black">.
You can read and write an object using serialization to any 
</FONT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black"><B>DataInputStream</B></FONT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black">
or 
</FONT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black"><B>DataOutputStream</B></FONT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black">
including, as you will see in the networking chapter, a network. The output
from one run was:
</FONT><P></DIV>

<font color="#990000"><PRE>Worm constructor: 6
Worm constructor: 5
Worm constructor: 4
Worm constructor: 3
Worm constructor: 2
Worm constructor: 1
w = :a(262):b(100):c(396):d(480):e(316):f(398)
Worm storage, w2 = :a(262):b(100):c(396):d(480):e(316):f(398)
Worm storage, w3 = :a(262):b(100):c(396):d(480):e(316):f(398) </PRE></font><DIV ALIGN=LEFT><P></DIV><DIV ALIGN=LEFT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black">You
can see that the deserialized object really does contain all of the links that
were in the original object.
</FONT><P></DIV><DIV ALIGN=LEFT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black">Note
that no constructor, not even the default constructor, is called in the process
of deserializing a 
</FONT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black"><B>Serializable</B></FONT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black">
object. The entire object is restored by recovering data from the 
</FONT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black"><B>InputStream</B></FONT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black">.</FONT><P></DIV><DIV ALIGN=LEFT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black">Object
serialization is another Java 1.1<A NAME="Index1320"></A>
feature that is not part of the new 
</FONT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black"><B>Reader</B></FONT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black">
and 
</FONT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black"><B>Writer</B></FONT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black">
hierarchies, but instead uses the old 
</FONT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black"><B>InputStream</B></FONT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black">
and 
</FONT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black"><B>OutputStream</B></FONT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black">
hierarchies. Thus you might encounter situations in which you&#8217;re forced
to mix the two hierarchies.
</FONT><a name="_Toc408018639"></a><P></DIV>
<A NAME="Heading344"></A><H3 ALIGN=LEFT>
Finding
the class
</H3>
<DIV ALIGN=LEFT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black">You
might wonder what&#8217;s necessary for an object to be recovered from its
serialized state. For example, suppose you serialize an object and send it as a
file or through a network to another machine. Could a program on the other
machine reconstruct the object using only the contents of the file?
</FONT><P></DIV><DIV ALIGN=LEFT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black">The
best way to answer this question is (as usual) by performing an experiment. The
following file goes in the subdirectory for this chapter:
</FONT><P></DIV>

<font color="#990000"><PRE><font color="#009900">//: Alien.java</font>
<font color="#009900">// A serializable class</font>
<font color="#0000ff">import</font> java.io.*;

<font color="#0000ff">public</font> <font color="#0000ff">class</font> Alien <font color="#0000ff">implements</font> Serializable {
} <font color="#009900">///:~ </PRE></font></font><DIV ALIGN=LEFT><P></DIV><DIV ALIGN=LEFT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black">The
file that creates and serializes an 
</FONT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black"><B>Alien
</B></FONT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black">object
goes in the same directory:
</FONT><P></DIV>

<font color="#990000"><PRE><font color="#009900">//: FreezeAlien.java</font>
<font color="#009900">// Create a serialized output file</font>
<font color="#0000ff">import</font> java.io.*;

<font color="#0000ff">public</font> <font color="#0000ff">class</font> FreezeAlien {
  <font color="#0000ff">public</font> <font color="#0000ff">static</font> <font color="#0000ff">void</font> main(String[] args) 
      <font color="#0000ff">throws</font> Exception {
    ObjectOutput out = 
      <font color="#0000ff">new</font> ObjectOutputStream(
        <font color="#0000ff">new</font> FileOutputStream("file.x"));
    Alien zorcon = <font color="#0000ff">new</font> Alien();
    out.writeObject(zorcon); 
  }
} <font color="#009900">///:~ </PRE></font></font><DIV ALIGN=LEFT><P></DIV><DIV ALIGN=LEFT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black">Rather
than catching and handling exceptions, this program takes the quick and dirty
approach of passing the exceptions out of 
</FONT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black"><B>main(&#160;)</B></FONT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black">,
so they&#8217;ll be reported on the command line.
</FONT><P></DIV><DIV ALIGN=LEFT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black">Once
the program is compiled and run, copy the resulting 
</FONT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black"><B>file.x</B></FONT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black">
to a subdirectory called 
</FONT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black"><B>xfiles</B></FONT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black">,
where the following code goes:
</FONT><P></DIV>

<font color="#990000"><PRE><font color="#009900">//: ThawAlien.java</font>
<font color="#009900">// Try to recover a serialized file without the </font>
<font color="#009900">// class of object that's stored in that file.</font>
<font color="#0000ff">package</font> c10.xfiles;
<font color="#0000ff">import</font> java.io.*;

<font color="#0000ff">public</font> <font color="#0000ff">class</font> ThawAlien {
  <font color="#0000ff">public</font> <font color="#0000ff">static</font> <font color="#0000ff">void</font> main(String[] args) 
      <font color="#0000ff">throws</font> Exception {
    ObjectInputStream in =
      <font color="#0000ff">new</font> ObjectInputStream(
        <font color="#0000ff">new</font> FileInputStream("file.x"));
    Object mystery = in.readObject();
    System.out.println(
      mystery.getClass().toString());
  }
} <font color="#009900">///:~ </PRE></font></font><DIV ALIGN=LEFT><P></DIV><DIV ALIGN=LEFT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black">This
program opens the file and reads in the object 
</FONT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black"><B>mystery</B></FONT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black">
successfully. However, as soon as you try to find out anything about the object
&#8211; which requires the 
</FONT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black"><B>Class</B></FONT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black">
object for 
</FONT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black"><B>Alien</B></FONT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black">
&#8211; the Java Virtual Machine (JVM) cannot find 
</FONT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black"><B>Alien.class</B></FONT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black">
(unless it happens to be in the Classpath, which it shouldn&#8217;t be in this
example). You&#8217;ll get a 
</FONT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black"><B>ClassNotFoundException.</B></FONT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black">
(Once again, all evidence of alien life vanishes before proof of its existence
can be verified!)
</FONT><P></DIV><DIV ALIGN=LEFT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black">If
you expect to do much after you&#8217;ve recovered an object that has been
serialized, you must make sure that the JVM can find the associated 
</FONT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black"><B>.class</B></FONT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black">
file either in the local class path or somewhere on the Internet.
</FONT><a name="_Toc408018640"></a><P></DIV>
<A NAME="Heading345"></A><H3 ALIGN=LEFT>
Controlling
serialization
</H3>
<DIV ALIGN=LEFT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black">As
you can see, the default serialization mechanism is trivial to use. But what if
you have special needs? Perhaps you have special security issues and you
don&#8217;t want to serialize portions of your object, or perhaps it just
doesn&#8217;t make sense for one sub-object to be serialized if that part needs
to be created anew when the object is recovered.
</FONT><P></DIV><DIV ALIGN=LEFT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black">You
can <A NAME="Index1321"></A><A NAME="Index1322"></A>control
the process of serialization by implementing the <A NAME="Index1323"></A><A NAME="Index1324"></A></FONT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black"><B>Externalizable</B></FONT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black">
interface instead of the <A NAME="Index1325"></A><A NAME="Index1326"></A></FONT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black"><B>Serializable</B></FONT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black">
interface. The 
</FONT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black"><B>Externalizable</B></FONT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black">
interface extends the 
</FONT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black"><B>Serializable</B></FONT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black">
interface and adds two methods, <A NAME="Index1327"></A><A NAME="Index1328"></A></FONT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black"><B>writeExternal(&#160;)</B></FONT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black">
and <A NAME="Index1329"></A><A NAME="Index1330"></A></FONT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black"><B>readExternal(&#160;),</B></FONT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black">
that
<a name="which"></a>
are automatically called for your object during serialization and
deserialization so that you can perform your special operations.
</FONT><P></DIV><DIV ALIGN=LEFT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black">The
following example shows simple implementations of the 
</FONT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black"><B>Externalizable</B></FONT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black">
interface methods. Note that 
</FONT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black"><B>Blip1</B></FONT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black">
and 
</FONT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black"><B>Blip2</B></FONT><FONT FACE="Carmina Md BT" SIZE=3 COLOR="Black">
are nearly identical except for a subtle difference (see if you can discover it
by looking at the code):
</FONT><P></DIV>

<font color="#990000"><PRE><font color="#009900">//: Blips.java</font>
<font color="#009900">// Simple use of Externalizable &amp; a pitfall</font>
<font color="#0000ff">import</font> java.io.*;
<font color="#0000ff">import</font> java.util.*;

<font color="#0000ff">class</font> Blip1 <font color="#0000ff">implements</font> Externalizable {
  <font color="#0000ff">public</font> Blip1() {
    System.out.println("Blip1 Constructor");
  }
  <font color="#0000ff">public</font> <font color="#0000ff">void</font> writeExternal(ObjectOutput out)
      <font color="#0000ff">throws</font> IOException {
    System.out.println("Blip1.writeExternal");
  }
  <font color="#0000ff">public</font> <font color="#0000ff">void</font> readExternal(ObjectInput in)
     <font color="#0000ff">throws</font> IOException, ClassNotFoundException {
    System.out.println("Blip1.readExternal");
  }
}

<font color="#0000ff">class</font> Blip2 <font color="#0000ff">implements</font> Externalizable {
  Blip2() {
    System.out.println("Blip2 Constructor");
  }
  <font color="#0000ff">public</font> <font color="#0000ff">void</font> writeExternal(ObjectOutput out)