Guava – Caching 工具类

Guava通过一个接口LoadingCache<K,V>提供了一个非常强大的基于内存的缓存机制。值会自动加载到缓存中，它提供了许多实用的方法来满足缓存需求.

Caching 工具类声明

以下是com.google.common.cache.LoadingCache <K,V>接口的声明 −

@Beta
@GwtCompatible
public interface LoadingCache<K,V>
   extends Cache<K,V>, Function<K,V>

Caching 工具类函数

序号	函数和描述
1	`V apply(K key)` 已废弃。为满足函数接口而提供；使用get(K)或getUnchecked(K)代替.
2	`ConcurrentMap<K,V> asMap()` 返回存储在这个缓存中的条目的视图，作为一个线程安全的映射.
3	`V get(K key)` 返回该缓存中与键相关的值，如果有必要，先加载该值.
4	`ImmutableMap<K,V> getAll(Iterable<? extends K> keys)` 返回一个与键相关的值的映射，如果需要的话，创建或检索这些值.
5	`V getUnchecked(K key)` 返回该缓存中与键相关的值，如果有必要，先加载该值.
6	`void refresh(K key)` 为key加载一个新的值，可能是异步的.

LoadingCache 工具类示例

使用你选择的任何编辑器创建以下java程序，例如C:/ > Guava.

GuavaTester.java

import java.util.HashMap;
import java.util.Map;
import java.util.concurrent.ExecutionException;
import java.util.concurrent.TimeUnit;

import com.google.common.base.MoreObjects;
import com.google.common.cache.CacheBuilder;
import com.google.common.cache.CacheLoader;
import com.google.common.cache.LoadingCache;

public class GuavaTester {
   public static void main(String args[]) {

      //create a cache for employees based on their employee id
      LoadingCache<String, Employee> employeeCache = 
         CacheBuilder.newBuilder()
         .maximumSize(100)                             // maximum 100 records can be cached
         .expireAfterAccess(30, TimeUnit.MINUTES)      // cache will expire after 30 minutes of access
         .build(new CacheLoader<String, Employee>() {  // build the cacheloader

            @Override
            public Employee load(String empId) throws Exception {
               //make the expensive call
               return getFromDatabase(empId);
            } 
         });

      try {         
         //on first invocation, cache will be populated with corresponding
         //employee record
         System.out.println("Invocation #1");
         System.out.println(employeeCache.get("100"));
         System.out.println(employeeCache.get("103"));
         System.out.println(employeeCache.get("110"));

         //second invocation, data will be returned from cache
         System.out.println("Invocation #2");
         System.out.println(employeeCache.get("100"));
         System.out.println(employeeCache.get("103"));
         System.out.println(employeeCache.get("110"));

      } catch (ExecutionException e) {
         e.printStackTrace();
      }
   }

   private static Employee getFromDatabase(String empId) {

      Employee e1 = new Employee("Mahesh", "Finance", "100");
      Employee e2 = new Employee("Rohan", "IT", "103");
      Employee e3 = new Employee("Sohan", "Admin", "110");

      Map<String, Employee> database = new HashMap<String, Employee>();

      database.put("100", e1);
      database.put("103", e2);
      database.put("110", e3);

      System.out.println("Database hit for" + empId);

      return database.get(empId);       
   }
}

class Employee {
   String name;
   String dept;
   String emplD;

   public Employee(String name, String dept, String empID) {
      this.name = name;
      this.dept = dept;
      this.emplD = empID;
   }

   public String getName() {
      return name;
   }

   public void setName(String name) {
      this.name = name;
   }

   public String getDept() {
      return dept;
   }

   public void setDept(String dept) {
      this.dept = dept;
   }

   public String getEmplD() {
      return emplD;
   }

   public void setEmplD(String emplD) {
      this.emplD = emplD;
   }

   @Override
   public String toString() {
      return MoreObjects.toStringHelper(Employee.class)
      .add("Name", name)
      .add("Department", dept)
      .add("Emp Id", emplD).toString();
   }    
}

Caching 工具类验证结果

使用javac编译器编译该类，如下所示 −

C:\Guava>javac GuavaTester.java

现在运行GuavaTester来看看结果.

C:\Guava>java GuavaTester

查看结果.

Invocation #1
Database hit for100
Employee{Name=Mahesh, Department=Finance, Emp Id=100}
Database hit for103
Employee{Name=Rohan, Department=IT, Emp Id=103}
Database hit for110
Employee{Name=Sohan, Department=Admin, Emp Id=110}
Invocation #2
Employee{Name=Mahesh, Department=Finance, Emp Id=100}
Employee{Name=Rohan, Department=IT, Emp Id=103}
Employee{Name=Sohan, Department=Admin, Emp Id=110}