perf4.html

jdbc书

<H3>Caching Many Objects</H3>

Sometimes you will want to cache more than one object. For example,
you might want to keep the most recently accessed files on a web server
in a cache.  If you use a <CODE>HashMap</CODE> object for a purpose
like this, it will continue to grow and use a lot of memory. 

<P>
If your machine has large amounts of memory and only a small number of objects
to cache then a growing <CODE>HashMap</CODE> may not be a problem.
However, if you are intending to cache alot of objects then you may find
that keeping only the most recent 
objects in the cache provides the best use of the machines memory. You can combine a <CODE>HashMap</CODE> object with a
<CODE>LinkedList</CODE> to create what is called a Most Recently Used
(MRU) cache.
<P>
<HR>
<BLOCKQUOTE>
<STRONG>Note:</STRONG> There are other techniques used to constrain cache size besides MRU. MRU is one of the simpler algorithms.
</BLOCKQUOTE>
<HR>
<P> 
With an MRU cache, you can place a constraint on which objects remain
in cache, and thereby, control the size of the cache. There are three main 
operations that the MRU cache has to perform:

<UL>
<LI><FONT FACE="Verdana, Arial, Helvetica, sans-serif">If the cache is not full, new objects not already in the cache
are inserted at the head of the list. </FONT>

<P>
<LI><FONT FACE="Verdana, Arial, Helvetica, sans-serif">If the cache is not full and the object to be inserted already exists in 
the cache, it is moved to the head of the list.</FONT>

<P>
<LI><FONT FACE="Verdana, Arial, Helvetica, sans-serif">If the cache is full and a new object is to be inserted, the last object 
in the cache is removed and the new object is inserted at the head of 
the list.</FONT>
</UL>

This diagram shows how the <CODE>LinkedList</CODE> and 
<CODE>HashMap</CODE> work together to implement the operations
described above. A discussion of the diagram follows.

<P>
<IMG SRC="./Art/cache.gif">
<P>
<DIV ALIGN="CENTER"><STRONG>MRU Cache with LinkedList and HashMap</STRONG></DIV>

<P>
The <CODE>LinkedList</CODE> provides the queue mechanism, and the entries
in the <CODE>LinkedList</CODE> contain the key to the data in the <CODE>
HashMap</CODE>. To add a new entry to the front of the list, the 
<CODE>addFirst</CODE> method is called. 

<UL>
<LI><FONT FACE="Verdana, Arial, Helvetica, sans-serif">If the list is already full, the <CODE>removeLast</CODE> method
is called and the data entry is also removed 
from the <CODE>HashMap</CODE>.</FONT>

<P>
<LI><FONT FACE="Verdana, Arial, Helvetica, sans-serif">
If an entry was already in the list, it is removed with
a call to the <CODE>remove</CODE> method and inserted at the front of the list with
a call to the <CODE>addFirst</CODE> method. </FONT>
</UL>

The Collections API does not implement locking, so if you remove entries
from or add entries to <CODE>LinkedList</CODE> or <CODE>HashMap</CODE>
objects, you need to 
lock access to these objects. You can also use a <CODE>Vector</CODE> or 
<CODE>ArrayList</CODE> to get the same results as shown in the code
below with the <CODE>LinkedList</CODE>.

<P>
This <A HREF="./Code/MRUCache.java">code example</A>
uses an MRU cache to keep a cache of files loaded from disk. 
When a file is requested, the program checks to see if the file
is in the cache. 
If the file is not in the cache, the program reads 
the file from disk and places the cache copy at the
beginning of the list. 

<P>
If the file is in cache, the program compares the 
modification times of the file and cache entry.

<UL>
<LI><FONT FACE="Verdana, Arial, Helvetica, sans-serif">If the cache entry time is older, the 
program reads the file from disk, removes the cache copy, and places
a new copy in the cache at the front of the <CODE>LinkedList</CODE>.</FONT>

<P>
<LI><FONT FACE="Verdana, Arial, Helvetica, sans-serif">If the file time is older, the program gets the file from 
the cache and moves the cache copy to the front of the list. </FONT>
</UL>

</FONT>

<PRE>
import java.util.*;
import java.io.*;

class myFile {
  long lastmodified;
  String contents;

  public myFile(long last, String data) {
    lastmodified=last;
    contents=data;		
  }
  public long getLastModified() {
    return lastmodified;
  }
  public String getContents() {
    return contents;
  }
}

public class MRUCache {

  Map cache;
  LinkedList mrulist;
  int cachesize;

  public MRUCache(int max) {
    cache = new HashMap();
    mrulist= new LinkedList();
    cachesize=max;
  }

  public String getFile(String fname) {
    if(!cache.containsKey(fname)) {
      synchronized(cache) {	
        if(mrulist.size() &gt;=cachesize) {
	  cache.remove(mrulist.getLast());
	  mrulist.removeLast();
        }
        cache.put(fname, readFile(fname)); 
        mrulist.addFirst(fname);
      }
    } else {
      if((new File(fname).lastModified())&gt; 
	((myFile)cache.get(fname)).getLastModified()) {
	  synchronized(cache) {	
	    cache.put(fname, readFile(fname)); 
          }
       }	
       synchronized(cache) {
	 mrulist.remove(fname);
	 mrulist.addFirst(fname);
       }
    }
       return ((myFile)cache.get(fname)).getContents();
  }
	
  public myFile readFile(String name) {
    File f = new File(name);
    StringBuffer filecontents= new StringBuffer();

    try {
      BufferedReader br=new BufferedReader(
                              new FileReader(f));
      String line;

      while((line =br.readLine()) != null) {
	filecontents.append(line);
      }	
    } catch (FileNotFoundException fnfe){
      return (null);
    } catch ( IOException ioe) {
		return (null);
    }
      return (new myFile(f.lastModified(), 
		filecontents.toString()));
  }

  public void printList() {
    for(int i=0;i&lt;mrulist.size();i++) {
      System.out.println("item "+i+"="+mrulist.get(i));
    }
  }

  public static void main(String args[]) {

    // Number of entries in MRU cache is set to 10 
    MRUCache h1=new MRUCache(10);
    for(int i=1;i&lt;=20;i++) {
      // files are stored in a subdirectory called data
      h1.getFile("data"+File.separatorChar+i);
    }
      h1.printList();
  }
}
</PRE>

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