File Changed Cache

讀取某個檔案建立model，可能因為成本昂貴而建立Cache；此外，程式可能會因為檔案有修改而重新建立Cache。本篇教學，將告訴你如何透過Guava達到這個需求。

Guava本身有提供ExpiringMemoizingSupplier讓你以時間為條件，去重讀cache；ExpiredFileMemorizeSupplier使用Proxy的概念，去控管何時該真的去讀實際資料。我參考這個做法，建立了ExpiredFileMemorizeSupplier物件，會根據檔案是否修改，去決定是否讀真實資料:

package org.tonylin.practice.guava.cache;
 
import java.io.File;
 
import com.google.common.base.Supplier;
 
public class ExpiredFileMemorizeSupplier<T> implements Supplier<T> {
 
	private File mFile;
	private Supplier<T> mDelegate;
	private long mLastModified = 0;
	private transient volatile T value;
 
	public ExpiredFileMemorizeSupplier(IFileDataSupplier<T> delegate){
		mFile = delegate.getFile();
		mDelegate = delegate;
	}
 
	public ExpiredFileMemorizeSupplier(Supplier<T> delegate, File file){
		mFile = file;
		mDelegate = delegate;
	}
 
	@Override
	public T get() {
		long lastModified = mFile.lastModified(); 
		synchronized (this) {
			if( lastModified != mLastModified ) {
				mLastModified = lastModified;
				value = mDelegate.get(); 
			}
			return value;	
		}
	}
}

通常提供資料的物件，本身應為對應檔案的Information Export；因此我另外定義IFileDataSupplier介面去做這事情:

package org.tonylin.practice.guava.cache;
 
import java.io.File;
 
import com.google.common.base.Supplier;
 
public interface IFileDataSupplier<T> extends Supplier<T> {
	File getFile();
}

以下為我的測試案例，去驗證資料是從cache來還是實際讀: (這延續之前的測試修改的，並非量身打造，莫見怪)

package org.tonylin.practice.guava.cache;
 
import java.io.File;
import java.io.IOException;
import java.net.MalformedURLException;
import java.nio.charset.Charset;
import java.util.concurrent.TimeUnit;
 
import org.junit.After;
import org.junit.Assert;
import org.junit.Before;
import org.junit.Test;
 
import com.google.common.base.Supplier;
import com.google.common.base.Suppliers;
import com.google.common.io.Files;
import com.google.common.io.Resources;
 
 
public class TestSuppliers {
 
	private int count = 0;
 
	private File testFile = new File("TestSuppliersTestFile.txt");
 
	@Before
	public void setup() throws Exception {
		Files.write("test".getBytes(), testFile);
	}
 
	@After
	public void teardown(){
		testFile.delete();
	}
 
	@Test
	public void testExpiredFileMemorizeSupplier_IFileDataSupplier() throws Exception {
		IFileDataSupplier<String> fileDataSupplier = new IFileDataSupplier<String>() {
			private File testFile = new File("TestSuppliersTestFile.txt");
 
			@Override
			public String get() {
				try {
					count++;
					return Resources.toString(testFile.toURI().toURL(), Charset.forName("utf-8"));
				} catch (Exception e) {
					throw new RuntimeException(e);
				}
			}
 
			@Override
			public File getFile() {
				return testFile;
			}
		};
 
		ExpiredFileMemorizeSupplier<String> s = new ExpiredFileMemorizeSupplier<>(fileDataSupplier);
 
		Assert.assertEquals( 0, count);
		Assert.assertEquals("test", s.get());
		Assert.assertEquals( 1, count);
		Assert.assertEquals("test", s.get());
		Assert.assertEquals( 1, count);
 
		Files.write("test2".getBytes(), testFile);
		Assert.assertEquals("test2", s.get());
		Assert.assertEquals( 2, count);
	}
 
	@Test
	public void testExpiredFileMemorizeSupplier() throws Exception{
		ExpiredFileMemorizeSupplier<String> s = new ExpiredFileMemorizeSupplier<>(this::getMemoize, testFile);
 
		Assert.assertEquals( 0, count);
		Assert.assertEquals("testMemoize", s.get());
		Assert.assertEquals( 1, count);
		Assert.assertEquals("testMemoize", s.get());
		Assert.assertEquals( 1, count);
 
		Files.write("test".getBytes(), testFile);
		Assert.assertEquals("testMemoize", s.get());
		Assert.assertEquals( 2, count);
	}
 
	public String getMemoize(){
		count++;
		return "testMemoize";
	}
}

也許應該設計成Event-based的Eviction?

在Linux上發現，如果修改時間間格非常短(1秒以下)有可能偵測不出來。如果不在意或不影響的就維持目前作法；如果在意可能就要改用File WatchService。

• Guava - CachesExplained

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