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Guava - CACHING UTILITIES

Guava provides a very powerful memory based caching mechanism by an interface LoadingCache<K,V>. Values are automatically loaded in the cache and it provides many utility methods useful for caching needs.

Interface Declaration

Following is the declaration for com.google.common.cache.LoadingCache<K,V> interface:

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

Interface Methods

S.N.	Method & Description
1	V apply(K key) Deprecated. Provided to satisfy the Function interface; use get(K) or getUnchecked(K) instead.
2	ConcurrentMap<K,V> asMap() Returns a view of the entries stored in this cache as a thread-safe map.
3	V get(K key) Returns the value associated with key in this cache, first loading that value if necessary.
4	ImmutableMap<K,V> getAll(Iterable<? extends K> keys) Returns a map of the values associated with keys, creating or retrieving those values if necessary.
5	V getUnchecked(K key) Returns the value associated with key in this cache, first loading that value if necessary.
6	void refresh(K key) Loads a new value for key, possibly asynchronously.

Example of Loading Cache

Create the following java program using any editor of your choice in say 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();
}
}

Verify the Result

Compile the class using javac compiler as follows:

C:\Guava>javac GuavaTester.java

Now run the GuavaTester to see the result.

C:\Guava>java GuavaTester

See the result.

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}

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Guava - CACHING UTILITIES