Guava 简明教程

Guava - Caching Utilities

Guava 通过一个 LoadingCache<K,V> 接口提供了非常强大的基于内存的缓存机制。值会自动加载到缓存中，它提供了许多对缓存需求有用的实用方法。

Interface Declaration

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>

Interface Methods

Sr.No	Method & Description
1	V apply(K key) 已弃用的。为了满足函数接口而提供；使用 get(K) 或 getUnchecked(K) 代替。
2	ConcurrentMap<K,V> asMap() 返回一个视图，其中存储在此缓存中的项以线程安全的方式映射。
3	V get(K key) 返回与此缓存中的键关联的值，如有必要，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，如有必要，则创建或检索这些值。
5	V getUnchecked(K key) 返回与此缓存中的键关联的值，如有必要，首先加载该值。
6	void refresh(K key) 加载键的新值，可能异步。

Example of LoadingCache

使用任意你选择的编辑器在 C:/> Guava. 中创建以下 Java 程序

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();
   }
}

Verify the Result

按照如下方式使用 javac 编译器编译类 −

C:\Guava>javac GuavaTester.java

现在运行 GuavaTester 以查看结果。

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}