gecko-dev/netwerk/test/TestDiskCache.js

var clientID = "javascript";
var key = "theme:button";
var nsICache = Components.interfaces.nsICache;

function getCacheService()
{
    var nsCacheService = Components.classes["@mozilla.org/network/cache-service;1"];
    var service = nsCacheService.getService(Components.interfaces.nsICacheService);
    return service;
}

function createCacheSession(clientID, storagePolicy, streamable)
{
    var service = getCacheService();
    var session = service.createSession(clientID, storagePolicy, streamable);
    return session;
}

function openCacheEntry(mode)
{
    var session = createCacheSession(clientID, nsICache.STORE_ON_DISK, true);
    var entry = session.openCacheEntry(key, mode);
    return entry;
}

function dumpLeaks()
{
    var leakDetector = Components.classes["@mozilla.org/xpcom/leakdetector;1"].getService(Components.interfaces.nsILeakDetector);
    leakDetector.dumpLeaks();
}

function wrapInputStream(input)
{
    var nsIScriptableInputStream = Components.interfaces.nsIScriptableInputStream;
    var factory = Components.classes["@mozilla.org/scriptableinputstream;1"];
    var wrapper = factory.createInstance(nsIScriptableInputStream);
    wrapper.init(input);
    return wrapper;
}

var outputEntry = openCacheEntry(nsICache.ACCESS_WRITE);
var output = outputEntry.transport.openOutputStream(0, -1, 0);
if (output.write("foo", 3) == 3)
    print("disk cache write works!");
else
    print("disk cache write broken!");

// store some metadata.
outputEntry.setMetaDataElement("size", "3");

output.close();
outputEntry.markValid();
outputEntry.close();

var inputEntry = openCacheEntry(nsICache.ACCESS_READ);
var input = wrapInputStream(inputEntry.transport.openInputStream(0, -1, 0));

if (input.read(input.available()) == "foo")
    print("disk cache read works!");
else
    print("disk cache read broken!");

if (inputEntry.getMetaDataElement("size") == "3")
    print("disk cache metadata works!");
else
    print("disk cache metadata broken!");

input.close();
inputEntry.close();