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如何从 HashMap 中删除过期元素并同时添加更多元素 - Java Timer、TimerTask 和 futures() - 完整示例

已发表: 2021-04-02
如何从 HashMap 中删除过期元素并同时添加更多元素 - Java Timer、TimerTask 和 futures()

Hashmap、ArrayList、Static Map、Vectors等是最常用的Java集合框架元素。 您可以根据需要使用无数种场景。

这个例子是非常有趣的Java Example。 我们将对单个 HashMap() 执行以下操作。

  • 创建 crunchifyMap 对象
  • 每秒继续向 Map 添加元素，过期时间设置为5 seconds
  • 每秒Check一次过期元素，如缓存delete from map if expired
  • 5 秒后，您将always same size和always 5 elements ，因为您每秒添加和删除过期元素

此外，如果您有below questions ，那么您来对地方了：

  • 什么是被动过期地图示例
  • 具有超时元素的并发映射
  • Java 缓存映射示例
  • Java TimerTask 示例
  • Evictor – Java 并发映射示例

让我们开始吧：

第 1 点

  • 创建定时器元素crunchifyTimer
  • 安排指定的任务CrunchifyReminder()重复固定延迟执行，即 1 秒
  • 在计划任务中
    • add元素添加到 crunchifyMap
    • 从 crunchifyMap 检查过期元素并删除

第 2 点

  • 在addElement()操作期间
    • 我们为每个元素关联current time
  • 在crunchifyClearExipredElementsFromMap()操作期间
    • 我们正在使用元素的时间检查当前时间
    • 如果时间差超过 5 秒，则只需从 crunchifyMap 中删除元素

第 3 点

  • 在 Eclipse 控制台上添加和删除操作打印元素期间
  • 第一个添加的元素将在第一个被删除，依此类推
  • 请检查 Eclipse 控制台输出的结果

这是一个Java程序：

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package crunchify . com . tutorials ;
 
import java . util . ArrayList ;
import java . util . Date ;
import java . util . HashMap ;
import java . util . Iterator ;
import java . util . Map ;
import java . util . Map . Entry ;
import java . util . Timer ;
import java . util . TimerTask ;
 
/**
* @author Crunchify.com
* <p>
* - Keep adding element to Map every second which has expire time set to 5 seconds
* - Check for expired element every second and delete from map if expired
* - After 5 seconds you will get always same size as you are adding and deleting expired elements every second
*/
 
public class CrunchifyCleanExipredMapElements {
Timer crunchifyTimer ;
private static final long EXPIRED_TIME_IN_SEC = 5L ;
private static final Map < Double , ArrayList < Date > > crunchifyMap = new HashMap < > ( ) ;
public static void main ( String [ ] args ) {
new CrunchifyCleanExipredMapElements ( 1 ) ;
crunchifyLog ( "Start Adding element every second\n\n" ) ;
}
public CrunchifyCleanExipredMapElements ( int seconds ) {
// Timer() - Creates a new timer. The associated thread does not run as a daemon.
crunchifyTimer = new Timer ( ) ;
// Schedules the specified task for repeated fixed-delay execution
// schedule(): Schedules the specified task for repeated fixed-delay execution, beginning after the specified delay.
// Subsequent executions take place at approximately regular intervals separated by the specified period.
// In fixed-delay execution, each execution is scheduled relative to the actual execution time of the previous execution.
// If an execution is delayed for any reason (such as garbage collection or other background activity), subsequent executions will be delayed as well.
// In the long run, the frequency of execution will generally be slightly lower than the reciprocal of the specified period (assuming the system clock underlying Object.wait(long) is accurate).
crunchifyTimer . schedule ( new CrunchifyReminder ( ) , 0 , seconds * 1000L ) ;
}
// TimeTask: A task that can be scheduled for one-time or repeated execution by a Timer.
// A timer task is not reusable. Once a task has been scheduled for execution on a Timer or cancelled,
// subsequent attempts to schedule it for execution will throw IllegalStateException.
class CrunchifyReminder extends TimerTask {
public void run ( ) {
// We are checking for expired element from map every second
crunchifyClearExpiredElementsFromMap ( ) ;
// We are adding element every second
addElement ( ) ;
}
}
public void addElement ( ) {
crunchifyAddElementToMap ( Math . random ( ) ) ;
}
// Check for element's expired time. If element is > 5 seconds old then remove it
private static void crunchifyClearExpiredElementsFromMap ( ) {
// Date() - Allocates a Date object and initializes it so that it represents the time at which it was allocated, measured to the nearest millisecond.
Date currentTime = new Date ( ) ;
Date actualExpiredTime = new Date ( ) ;
// if element time stamp and current time stamp difference is 5 second then delete element
actualExpiredTime . setTime ( currentTime . getTime ( ) - EXPIRED_TIME_IN_SEC * 1000L ) ;
// size() - Returns the number of key-value mappings in this map. If the map contains more than Integer.MAX_VALUE elements, returns Integer.MAX_VALUE.
System . out . println ( "crunchifyMap size:" + CrunchifyCleanExipredMapElements . crunchifyMap . size ( ) ) ;
Iterator < Entry < Double , ArrayList < Date > > > crunchifyIterator = CrunchifyCleanExipredMapElements . crunchifyMap . entrySet ( ) . iterator ( ) ;
// hasNext() - Returns true if the iteration has more elements. (In other words, returns true if next would return an element rather than throwing an exception.)
while ( crunchifyIterator . hasNext ( ) ) {
// next() - Returns the next element in the iteration.
Entry < Double , ArrayList < Date > > entry = crunchifyIterator . next ( ) ;
// getValue() - Returns the value corresponding to this entry.
// If the mapping has been removed from the backing map (by the iterator's remove operation), the results of this call are undefined.
ArrayList < Date > crunchifyElement = entry . getValue ( ) ;
while ( crunchifyElement . size ( ) > 0
&& crunchifyElement.get(0).compareTo(actualExpiredTime) < 0) {
crunchifyLog("----------- Expired Element Deleted: " + entry.getKey());
crunchifyElement . remove ( 0 ) ;
}
if ( crunchifyElement . size ( ) == 0 ) {
// remove() - Removes from the underlying collection the last element returned by this iterator (optional operation).
// This method can be called only once per call to next.
crunchifyIterator . remove ( ) ;
}
}
}
// Adding new element to map with current timestamp
private static void crunchifyAddElementToMap ( Double digit ) {
ArrayList < Date > crunchifyList = new ArrayList < > ( ) ;
CrunchifyCleanExipredMapElements . crunchifyMap . put ( digit , crunchifyList ) ;
// add() - Appends the specified element to the end of this list.
crunchifyList . add ( new Date ( ) ) ;
crunchifyLog ( "+++++++++++ Add Element:" + digit + "\n" ) ;
}
private static void crunchifyLog ( String string ) {
System . out . println ( string ) ;
}
}
 

Eclipse 控制台输出：

只需运行上面的程序，您应该会看到如下结果。

如何从 HashMap 中删除过期元素并同时添加更多元素 - Java Timer、TimerTask 和 futures() - 控制台结果
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Start Adding element every second
crunchifyMap size : 0
+++++++++++ Add Element : 0.6970683199424504
crunchifyMap size : 1
+++++++++++ Add Element : 0.8103548990859972
crunchifyMap size : 2
+++++++++++ Add Element : 0.849973176897403
crunchifyMap size : 3
+++++++++++ Add Element : 0.2770459184269851
crunchifyMap size : 4
+++++++++++ Add Element : 0.9136028332849369
crunchifyMap size : 5
----------- Expired Element Deleted : 0.6970683199424504
+++++++++++ Add Element : 0.6630362698520832
crunchifyMap size : 5
----------- Expired Element Deleted : 0.8103548990859972
+++++++++++ Add Element : 0.38906826918659854
crunchifyMap size : 5
----------- Expired Element Deleted : 0.849973176897403
+++++++++++ Add Element : 0.3997935729434138
crunchifyMap size : 5
----------- Expired Element Deleted : 0.2770459184269851
+++++++++++ Add Element : 0.20373441843273887
 
Process finished with exit code 130 ( interrupted by signal 2 : SIGINT )

如果您在 Java 程序上运行时遇到任何问题，请告诉我。