Servlet Assets Helper

example.ftl

<@resources.stylesheet "default.css" />

<@resources.javascript "jquery.min.js" />

resources.ftl

<#macro stylesheet path>
<link rel="stylesheet" href="${StaticResources.uri("/resources/" + path)}" type="text/css" />
</#macro>

<#macro javascript path>
<script src="${StaticResources.uri("/resources/" + path)}" type="text/javascript"></script>
</#macro>

StaticResources.java

import java.io.IOException;
import java.util.concurrent.TimeUnit;

import javax.servlet.ServletContext;

import com.google.common.base.Preconditions;
import com.google.common.cache.CacheBuilder;
import com.google.common.cache.CacheLoader;
import com.google.common.cache.LoadingCache;
import com.google.common.hash.HashFunction;
import com.google.common.hash.Hashing;
import com.google.common.io.Resources;

public class StaticResources {
	private HashFunction signatureFunction = Hashing.md5();
	private ServletContext context;
	private LoadingCache<String, String> signatures;
	
	public StaticResources(ServletContext context, boolean alwaysFresh) {
		Preconditions.checkNotNull(context, "The servlet context is required");
		
		this.context = context;
		CacheLoader<String, String> signatureCacheLoader = new CacheLoader<String, String>() {
			public String load(String key) {
				return signature(key);
			}
		};
		CacheBuilder<Object, Object> builder = CacheBuilder.newBuilder();
		if (alwaysFresh) {
			builder.expireAfterWrite(0, TimeUnit.SECONDS);
		}
		this.signatures = builder.build(signatureCacheLoader);
	}
	
	public String signature(String path) {
		try {
			return signatureFunction.hashBytes(Resources.toByteArray(context.getResource(path))).toString();
		} catch (IOException e) {
			throw new RuntimeException("Unable to read the resource", e);
		}
	}

	/**
	 * The path must begin with a "/" and is interpreted as relative to the current context root. 
	 */
	public String uri(String path) {
		Preconditions.checkArgument(path.startsWith("/"), "The path must begin with a \"/\".");
		StringBuilder result = new StringBuilder(context.getContextPath());
		result.append(path).append("?_s=").append(signatures.getUnchecked(path));
		return result.toString();
	}
}

StaticResourcesInitializer.java

import javax.servlet.ServletContext;

import org.springframework.beans.factory.InitializingBean;
import org.springframework.web.context.ServletContextAware;

public class StaticResourcesInitializer implements ServletContextAware, InitializingBean {
	private ServletContext context;
	private boolean alwaysFresh = false;
	
	public void setServletContext(ServletContext context) {
		this.context = context;
	}
	
	public void setAlwaysFresh(boolean alwaysFresh) {
		this.alwaysFresh = alwaysFresh;
	}
	
	@Override
	public void afterPropertiesSet() throws Exception {
		context.setAttribute("StaticResources", new StaticResources(context, alwaysFresh));
	}
}

StaticResourcesTest.java

import static org.junit.Assert.*;
import static org.mockito.Mockito.*;

import javax.servlet.ServletContext;

import org.junit.Before;
import org.junit.Test;
import org.junit.runner.RunWith;
import org.mockito.Mock;
import org.mockito.runners.MockitoJUnitRunner;

import com.google.common.io.Resources;

@RunWith(MockitoJUnitRunner.class)
public class StaticResourcesTest {
	private StaticResources dut;
	private @Mock ServletContext context;
	private String path = "/resources/jquery-1.6.1.min.js";
	private String sig = "9573f9134986b52cff0401ac3218d075";

	@Before public void beforeEach() throws Exception {
		dut = new StaticResources(context, true);
		when(context.getResource(path)).thenReturn(Resources.getResource("webapps" + path));
	}
	
	@Test public void signature() {
		assertEquals(sig, dut.signature(path));
	}
	
	@Test public void uri_with_root_context_path() {
		when(context.getContextPath()).thenReturn("");
		assertEquals(path + "?_s=" + sig, dut.uri(path));
	}
	
	@Test(expected=IllegalArgumentException.class) public void uri_throws_exception_when_the_path_does_not_start_with_slash() {
		when(context.getContextPath()).thenReturn("");
		assertEquals(path + "?_s=" + sig, dut.uri("resources/x.js"));
	}
	
	@Test public void uri_with_none_root_context_path() {
		when(context.getContextPath()).thenReturn("/login");
		assertEquals("/login" + path + "?_s=" + sig, dut.uri(path));
	}
}