tomcat 7 源码分析-12 Enumeration枚举-优快云博客

本文深入剖析了Tomcat7中用于高效且无重复地遍历HTTP头部名称的Enumeration实现机制。通过具体源码示例，详细解读了核心方法findNext()的工作原理及其如何确保返回的每个元素都是唯一的。

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tomcat 7 源码分析-12 Enumeration枚举

Enumeration枚举，就是要把内容没有重复的遍历，而且不破坏原有的存储空间。在http headers的实现中，tomcat实现了一些枚举类。

Enumerate the distinct header names. Each nextElement() is O(n)

Java代码

class NamesEnumerator implements Enumeration<String> {
int pos;
int size;
String next;
MimeHeaders headers;
public NamesEnumerator(MimeHeaders headers) {
this .headers=headers;
pos=0 ;
size = headers.size();
findNext();
}
private void findNext() {
next=null ;
for ( ; pos< size; pos++ ) {
next=headers.getName( pos ).toString();
for ( int j= 0 ; j<pos ; j++ ) {
if ( headers.getName( j ).equalsIgnoreCase( next )) {
// duplicate.
next=null ;
break ;
}
}
if ( next!= null ) {
// it's not a duplicate
break ;
}
}
// next time findNext is called it will try the
// next element
pos++;
}
public boolean hasMoreElements() {
return next!= null ;
}
public String nextElement() {
String current=next;
findNext();
return current;
}
}

class NamesEnumerator implements Enumeration<String> {
    int pos;
    int size;
    String next;
    MimeHeaders headers;

    public NamesEnumerator(MimeHeaders headers) {
        this.headers=headers;
        pos=0;
        size = headers.size();
        findNext();
    }

    private void findNext() {
        next=null;
        for(  ; pos< size; pos++ ) {
            next=headers.getName( pos ).toString();
            for( int j=0; j<pos ; j++ ) {
                if( headers.getName( j ).equalsIgnoreCase( next )) {
                    // duplicate.
                    next=null;
                    break;
                }
            }
            if( next!=null ) {
                // it's not a duplicate
                break;
            }
        }
        // next time findNext is called it will try the
        // next element
        pos++;
    }
    
    public boolean hasMoreElements() {
        return next!=null;
    }

    public String nextElement() {
        String current=next;
        findNext();
        return current;
    }
}

Enumeration的核心是findNext函数，实现了找到下一个元素的功能，保障与之前的不重复。

简单分析findNext的算法：

pos就表示next的位置，如果在循环pos前面的元素时，发现有重复的，就把next=null，跳出循环后，判断是否为null。

如果next为null，外边的循环仍然继续，pos后移，此举就是跳出有重复的元素。

如果next不等于null，说明此pos的元素与之前不重复，next即是下一个元素。整个循环结束。

最后pos++，为下一个next做准备。

类似的写了个队char的枚举类，验证了下算法

Java代码

package com.test.Enumerator;
import java.util.Enumeration;
public class CharEnumeration implements Enumeration<Character> {
Character next;
Character characters[];
int pos;
int size;
CharEnumeration(Character a[]) {
characters = a;
size = a.length;
pos = 0 ;
findNext();
}
@Override
public boolean hasMoreElements() {
// TODO Auto-generated method stub
return next != null ;
}
private void findNext() {
next = null ;
for (; pos < size; pos++) {
next = characters[pos];
for ( int i = 0 ; i < pos; i++) {
if (next == characters[i]) {
next = null ;
break ;
}
}
if (next != null )
break ;
}
pos++;
}
@Override
public Character nextElement() {
// TODO Auto-generated method stub
Character cur = next;
findNext();
return cur;
}
public void PrintAll() {
while (hasMoreElements()) {
System.out.println(nextElement());
}
}
}

package com.test.Enumerator;

import java.util.Enumeration;

public class CharEnumeration implements Enumeration<Character> {

	Character next;
	Character characters[];
	int pos;
	int size;

	CharEnumeration(Character a[]) {

		characters = a;
		size = a.length;
		pos = 0;
		findNext();
	}

	@Override
	public boolean hasMoreElements() {
		// TODO Auto-generated method stub
		return next != null;
	}

	private void findNext() {
		next = null;

		for (; pos < size; pos++) {

			next = characters[pos];
			for (int i = 0; i < pos; i++) {

				if (next == characters[i]) {
					next = null;
					break;
				}
			}

			if (next != null)
				break;
		}
		pos++;
	}

	@Override
	public Character nextElement() {
		// TODO Auto-generated method stub		
		Character cur = next;
		findNext();
		return cur;
	}

	public void PrintAll() {

		while (hasMoreElements()) {
			
			System.out.println(nextElement());
		}
	}

}

Java代码

package com.test.Enumerator;
public class EnumeratorTest {
/**
* @param args
*/
public static void main(String[] args) {
// TODO Auto-generated method stub
Character [] sample = {'a' , 'b' , 'c' , 'd' , 'e' , 'a' , 'a' , 'b' };
CharEnumeration che = new CharEnumeration(sample);
che.PrintAll();
}
}

package com.test.Enumerator;
public class EnumeratorTest {

	/**
	 * @param args
	 */
	public static void main(String[] args) {
		// TODO Auto-generated method stub

		Character [] sample = {'a','b','c','d','e','a','a','b'};
		CharEnumeration che = new CharEnumeration(sample);
		che.PrintAll();
	}

}