enable client side caching for plot requests

Doesn't work perfectly yet, because the height/width sometimes changes
by one or two pixels.

pdb-ui/src/main/java/org/lucares/pdbui/WebConfiguration.java

@@ -1,19 +1,25 @@
 package org.lucares.pdbui;
 
 import java.nio.file.Paths;
+import java.util.concurrent.TimeUnit;
 
 import org.springframework.beans.factory.annotation.Value;
 import org.springframework.context.annotation.Configuration;
+import org.springframework.http.CacheControl;
 import org.springframework.web.servlet.config.annotation.ResourceHandlerRegistry;
-import org.springframework.web.servlet.config.annotation.WebMvcConfigurerAdapter;
+import org.springframework.web.servlet.config.annotation.WebMvcConfigurer;
 
 @Configuration
-public class WebConfiguration extends WebMvcConfigurerAdapter implements HardcodedValues, PropertyKeys {
+public class WebConfiguration implements WebMvcConfigurer, HardcodedValues, PropertyKeys {
 
 	private final String outputDir;
+	private final int cacheDurationInSeconds;
 
-	public WebConfiguration(@Value("${" + PATH_GENERATED_IMAGES + "}") final String outputDir) {
+	public WebConfiguration(@Value("${" + PATH_GENERATED_IMAGES + "}") final String outputDir,
+			@Value("${" + CACHE_IMAGES_DURATION_SECONDS + ":" + CACHE_IMAGES_DURATION_SECONDS_DEFAULT
+					+ "}") final int cacheDurationInSeconds) {
 		this.outputDir = outputDir;
+		this.cacheDurationInSeconds = cacheDurationInSeconds;
 	}
 
 	@Override
@@ -22,6 +28,7 @@ public class WebConfiguration extends WebMvcConfigurerAdapter implements Hardcod
 		final String pathPattern = "/" + WEB_IMAGE_OUTPUT_PATH + "/**";
 		final String resourceLocation = "file:" + Paths.get(outputDir).toAbsolutePath() + "/";
 
-		registry.addResourceHandler(pathPattern).addResourceLocations(resourceLocation);
+		final CacheControl cacheControl = CacheControl.maxAge(cacheDurationInSeconds, TimeUnit.SECONDS);
+		registry.addResourceHandler(pathPattern).addResourceLocations(resourceLocation).setCacheControl(cacheControl);
 	}
 }