Java ConcurrentHashMap

V tomto tutoriále sa pomocou príkladov dozvieme o triede Java ConcurrentHashMap a jej operáciách.

ConcurrentHashMapTrieda zbierok rámca Java poskytuje thread-safe mapu. To znamená, že na mapu môže naraz pristupovať viac vlákien bez toho, aby to ovplyvnilo konzistenciu záznamov na mape.

Implementuje rozhranie ConcurrentMap.

Vytvorte ConcurrentHashMap

Aby sme vytvorili súbežnú hashmapu, musíme najskôr importovať java.util.concurrent.ConcurrentHashMapbalík. Po importovaní balíka tu môžeme vytvoriť súbežné hashmapy v Jave.

 // ConcurrentHashMap with capacity 8 and load factor 0.6 ConcurrentHashMap numbers = new ConcurrentHashMap(8, 0.6f);

Vo vyššie uvedenom kóde sme vytvorili súbežnú hashmapu pomenovanú čísla.

Tu,

  • Kľúč - jedinečný identifikátor, ktorý sa používa na priradenie každého prvku (hodnoty) k mape
  • Hodnota - prvky spojené s klávesmi na mape

Všimnite si časť new ConcurrentHashMap(8, 0.6). Tu je prvým parametrom kapacita a druhým parametrom je loadFactor .

  • kapacita - kapacita tejto mapy je 8. To znamená, že do nej možno uložiť 8 záznamov.
  • loadFactor - Faktor zaťaženia tejto mapy je 0,6. To znamená, že kedykoľvek je naša hash tabuľka naplnená o 60%, položky sú presunuté do novej hash tabuľky s dvojnásobnou veľkosťou ako pôvodná hash tabuľka.

Predvolená kapacita a faktor zaťaženia

Je možné vytvoriť súbežnú hashmapu bez definovania jej kapacity a koeficientu zaťaženia. Napríklad,

 // ConcurrentHashMap with default capacity and load factor ConcurrentHashMap numbers1 = new ConcurrentHashMap();

Predvolene,

  • kapacita mapy bude 16
  • činiteľ zaťaženia bude 0,75

Vytváranie ConcurrentHashMap z iných máp

Týmto spôsobom môžeme vytvoriť súbežnú hashmapu obsahujúcu všetky prvky ostatných máp.

 import java.util.concurrent.ConcurrentHashMap; import java.util.HashMap; class Main ( public static void main(String() args) ( // Creating a hashmap of even numbers HashMap evenNumbers = new HashMap(); evenNumbers.put("Two", 2); evenNumbers.put("Four", 4); System.out.println("HashMap: " + evenNumbers); // Creating a concurrent hashmap from other map ConcurrentHashMap numbers = new ConcurrentHashMap(evenNumbers); numbers.put("Three", 3); System.out.println("ConcurrentHashMap: " + numbers); ) )

Výkon

 HashMap: (štyri = 4, dva = 2) ConcurrentHashMap: (štyri = 4, dva = 2, tri = 3)

Metódy ConcurrentHashMap

ConcurrentHashMapTrieda poskytuje metódy, ktoré nám umožnia vykonávať rôzne operácie na mape.

Vložte prvky do ConcurrentHashMap

  • put() - vloží zadané mapovanie kľúča / hodnoty do mapy
  • putAll() - vloží všetky záznamy z určenej mapy na túto mapu
  • putIfAbsent() - vloží zadané mapovanie kľúča / hodnoty do mapy, ak sa uvedený kľúč na mape nenachádza

Napríklad,

 import java.util.concurrent.ConcurrentHashMap; class Main ( public static void main(String() args) ( // Creating ConcurrentHashMap of even numbers ConcurrentHashMap evenNumbers = new ConcurrentHashMap(); // Using put() evenNumbers.put("Two", 2); evenNumbers.put("Four", 4); // Using putIfAbsent() evenNumbers.putIfAbsent("Six", 6); System.out.println("ConcurrentHashMap of even numbers: " + evenNumbers); //Creating ConcurrentHashMap of numbers ConcurrentHashMap numbers = new ConcurrentHashMap(); numbers.put("One", 1); // Using putAll() numbers.putAll(evenNumbers); System.out.println("ConcurrentHashMap of numbers: " + numbers); ) )

Výkon

 ConcurrentHashMap párnych čísel: (šesť = 6, štyri = 4, dva = 2) ConcurrentHashMap čísel: (šesť = 6, jeden = 1, štyri = -4, dva = 2)

Prístup k prvkom ConcurrentHashMap

1. Pomocou položiek entrySet (), keySet () a values ​​()

  • entrySet() - vráti množinu všetkých mapovaní kľúčov / hodnôt mapy
  • keySet() - vráti sadu všetkých kľúčov mapy
  • values() - vráti množinu všetkých hodnôt mapy

Napríklad,

 import java.util.concurrent.ConcurrentHashMap; class Main ( public static void main(String() args) ( ConcurrentHashMap numbers = new ConcurrentHashMap(); numbers.put("One", 1); numbers.put("Two", 2); numbers.put("Three", 3); System.out.println("ConcurrentHashMap: " + numbers); // Using entrySet() System.out.println("Key/Value mappings: " + numbers.entrySet()); // Using keySet() System.out.println("Keys: " + numbers.keySet()); // Using values() System.out.println("Values: " + numbers.values()); ) )

Výkon

 ConcurrentHashMap: (Jeden = 1, Dva = 2, Tri = 3) Mapovanie kľúča / hodnoty: (Jeden = 1, Dva = 2, Tri = 3) Kľúče: (Jeden, Dva, Tri) Hodnoty: (1, 2, 3 )

2. Používanie get () a getOrDefault ()

  • get()- Vráti hodnotu spojenú so zadaným kľúčom. Vráti sa, nullak kľúč nenájdete.
  • getOrDefault()- Vráti hodnotu spojenú so zadaným kľúčom. Vráti zadanú predvolenú hodnotu, ak sa kľúč nenájde.

Napríklad,

 import java.util.concurrent.ConcurrentHashMap; class Main ( public static void main(String() args) ( ConcurrentHashMap numbers = new ConcurrentHashMap(); numbers.put("One", 1); numbers.put("Two", 2); numbers.put("Three", 3); System.out.println("ConcurrentHashMap: " + numbers); // Using get() int value1 = numbers.get("Three"); System.out.println("Using get(): " + value1); // Using getOrDefault() int value2 = numbers.getOrDefault("Five", 5); System.out.println("Using getOrDefault(): " + value2); ) )

Výkon

 ConcurrentHashMap: (Jeden = 1, Dva = 2, Tri = 3) Použitie get (): 3 Pomocou getOrDefault (): 5

Odstrániť prvky ConcurrentHashMap

  • remove(key) - vráti a odstráni z mapy položku spojenú so zadaným kľúčom
  • remove(key, value) - removes the entry from the map only if the specified key mapped to the specified value and return a boolean value

For example,

 import java.util.concurrent.ConcurrentHashMap; class Main ( public static void main(String() args) ( ConcurrentHashMap numbers = new ConcurrentHashMap(); numbers.put("One", 1); numbers.put("Two", 2); numbers.put("Three", 3); System.out.println("ConcurrentHashMap: " + numbers); // remove method with single parameter int value = numbers.remove("Two"); System.out.println("Removed value: " + value); // remove method with two parameters boolean result = numbers.remove("Three", 3); System.out.println("Is the entry (Three=3) removed? " + result); System.out.println("Updated ConcurrentHashMap: " + numbers); ) )

Output

 ConcurrentHashMap: (One=1, Two=2, Three=3) Removed value: 2 Is the entry (Three=3) removed? True Updated ConcurrentHashMap: (One=1)

Bulk ConcurrentHashMap Operations

The ConcurrentHashMap class provides different bulk operations that can be applied safely to concurrent maps.

1. forEach() Method

The forEach() method iterates over our entries and executes the specified function.

It includes two parameters.

  • parallelismThreshold - It specifies that after how many elements operations in a map are executed in parallel.
  • transformer - This will transform the data before the data is passed to the specified function.

For example,

 import java.util.concurrent.ConcurrentHashMap; class Main ( public static void main(String() args) ( ConcurrentHashMap numbers = new ConcurrentHashMap(); numbers.put("One", 1); numbers.put("Two", 2); numbers.put("Three", 3); System.out.println("ConcurrentHashMap: " + numbers); // forEach() without transformer function numbers.forEach(4, (k, v) -> System.out.println("key: " + k + " value: " + v)); // forEach() with transformer function System.out.print("Values are "); numbers.forEach(4, (k, v) -> v, (v) -> System.out.print(v + ", ")); ) )

Output

 ConcurrentHashMap: (One = 1, Two = 2, Three = 3) key: One value: 1 key: Two value: 2 key: Three value: 3 Values are 1, 2, 3,

In the above program, we have used parallel threshold 4. This means if the map contains 4 entries, the operation will be executed in parallel.

Variation of forEach() Method

  • forEachEntry() - executes the specified function for each entry
  • forEachKey() - executes the specified function for each key
  • forEachValue() - executes the specified function for each value

2. search() Method

The search() method searches the map based on the specified function and returns the matched entry.

Here, the specified function determines what entry is to be searched.

It also includes an optional parameter parallelThreshold. The parallel threshold specifies that after how many elements in the map the operation is executed in parallel.

For example,

 import java.util.concurrent.ConcurrentHashMap; class Main ( public static void main(String() args) ( ConcurrentHashMap numbers = new ConcurrentHashMap(); numbers.put("One", 1); numbers.put("Two", 2); numbers.put("Three", 3); System.out.println("ConcurrentHashMap: " + numbers); // Using search() String key = numbers.search(4, (k, v) -> (return v == 3 ? k: null;)); System.out.println("Searched value: " + key); ) )

Output

 ConcurrentHashMap: (One=1, Two=2, Three=3) Searched value: Three

Variants of search() Method

  • searchEntries() - search function is applied to key/value mappings
  • searchKeys() - search function is only applied to the keys
  • searchValues() - search function is only applied to the values

3. reduce() Method

The reduce() method accumulates (gather together) each entry in a map. This can be used when we need all the entries to perform a common task, like adding all the values of a map.

It includes two parameters.

  • parallelismThreshold - It specifies that after how many elements, operations in a map are executed in parallel.
  • transformer - This will transform the data before the data is passed to the specified function.

For example,

 import java.util.concurrent.ConcurrentHashMap; class Main ( public static void main(String() args) ( ConcurrentHashMap numbers = new ConcurrentHashMap(); numbers.put("One", 1); numbers.put("Two", 2); numbers.put("Three", 3); System.out.println("ConcurrentHashMap: " + numbers); // Using search() int sum = numbers.reduce(4, (k, v) -> v, (v1, v2) -> v1 + v2); System.out.println("Sum of all values: " + sum); ) )

Output

 ConcurrentHashMap: (One=1, Two=2, Three=3) Sum of all values: 6

In the above program, notice the statement

 numbers.reduce(4, (k, v) -> v, (v1, v2) -> v1+v2);

Here,

  • 4 is a parallel threshold
  • (k, v) -> v is a transformer function. It transfers the key/value mappings into values only.
  • (v1, v2) -> v1+v2 is a reducer function. It gathers together all the values and adds all values.

Variants of reduce() Method

  • reduceEntries() - returns the result of gathering all the entries using the specified reducer function
  • reduceKeys() - returns the result of gathering all the keys using the specified reducer function
  • reduceValues() - returns the result of gathering all the values using the specified reducer function

ConcurrentHashMap vs HashMap

Here are some of the differences between ConcurrentHashMap and HashMap,

  • ConcurrentHashMap is a thread-safe collection. That is, multiple threads can access and modify it at the same time.
  • ConcurrentHashMap provides methods for bulk operations like forEach(), search() and reduce().

Why ConcurrentHashMap?

  • ConcurrentHashMapTrieda umožňuje viac vlákien prístup k jeho záznamov súčasne.
  • Štandardne je súbežná hashmap rozdelená do 16 segmentov . Z tohto dôvodu môže 16 vlákien súčasne upravovať mapu. K mape má však prístup ľubovoľný počet vlákien súčasne.
  • putIfAbsent()Metóda nebude prepísať položky v mape, ak je zadaný kľúč už existuje.
  • Poskytuje vlastnú synchronizáciu.

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