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2. 学习guava
3. guava - 缓存实用程序( Caching Utilities)

guava - 缓存实用程序( Caching Utilities)

Guava通过一个接口LoadingCache 提供了一个非常强大的基于内存的缓存机制。 值自动加载到缓存中，它提供了许多对缓存需求有用的实用方法。

接口声明 (Interface Declaration)

以下是com.google.common.cache.LoadingCache《K,V》界面的声明 -

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

接口方法

Sr.No	方法和描述
1	V apply(K key) 已过时。 提供满足功能界面; 请改用get（K）或getUnchecked（K）。
2	ConcurrentMap《K,V》 asMap() 返回存储在此缓存中的条目视图作为线程安全映射。
3	V get(K key) 返回与此缓存中的键关联的值，必要时首先加载该值。
4	ImmutableMap《K,V》 getAll(Iterable《? extends K》 keys) 返回与键关联的值的映射，必要时创建或检索这些值。
5	V getUnchecked(K key) 返回与此缓存中的键关联的值，必要时首先加载该值。
6	void refresh(K key) 为key加载一个新值，可能是异步的。

LoadingCache示例

使用您选择的任何编辑器在C:/》 Guava.创建以下java程序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();
   }	
}

验证结果

使用javac编译器编译类如下 -

C:\Guava>javac GuavaTester.java

现在运行GuavaTester来查看结果。

C:\Guava>java GuavaTester

看到结果。

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}