diff --git a/runelite-client/src/main/java/net/runelite/client/rs/ClientLoader.java b/runelite-client/src/main/java/net/runelite/client/rs/ClientLoader.java
index fb04be2ec2..10c9f76bc8 100644
--- a/runelite-client/src/main/java/net/runelite/client/rs/ClientLoader.java
+++ b/runelite-client/src/main/java/net/runelite/client/rs/ClientLoader.java
@@ -30,6 +30,7 @@ import com.google.archivepatcher.applier.FileByFileV1DeltaApplier;
 import com.google.common.base.Strings;
 import com.google.common.hash.Hashing;
 import com.google.common.hash.HashingOutputStream;
+import com.google.common.io.ByteStreams;
 import com.google.common.io.Files;
 import java.applet.Applet;
 import java.io.ByteArrayOutputStream;
@@ -39,8 +40,6 @@ import java.io.FileOutputStream;
 import java.io.IOException;
 import java.io.InputStream;
 import java.io.OutputStream;
-import java.net.URL;
-import java.net.URLClassLoader;
 import java.nio.channels.Channels;
 import java.nio.channels.FileChannel;
 import java.nio.channels.FileLock;
@@ -51,8 +50,10 @@ import java.security.cert.CertificateFactory;
 import java.util.Arrays;
 import java.util.Collection;
 import java.util.Map;
+import java.util.Enumeration;
 import java.util.function.Supplier;
 import java.util.jar.JarEntry;
+import java.util.jar.JarFile;
 import java.util.jar.JarInputStream;
 import javax.swing.SwingUtilities;
 import lombok.extern.slf4j.Slf4j;
@@ -120,6 +121,7 @@ public class ClientLoader implements Supplier<Applet>
 			SplashScreen.stage(.05, null, "Waiting for other clients to start");
 
 			LOCK_FILE.getParentFile().mkdirs();
+			ClassLoader classLoader;
 			try (FileChannel lockfile = FileChannel.open(LOCK_FILE.toPath(),
 				StandardOpenOption.CREATE, StandardOpenOption.READ, StandardOpenOption.WRITE);
 				FileLock flock = lockfile.lock())
@@ -132,14 +134,17 @@ public class ClientLoader implements Supplier<Applet>
 					SplashScreen.stage(.35, null, "Patching");
 					applyPatch();
 				}
-			}
 
-			File jarFile = updateCheckMode == AUTO ? PATCHED_CACHE : VANILLA_CACHE;
-			URL jar = jarFile.toURI().toURL();
+				SplashScreen.stage(.40, null, "Loading client");
+				File jarFile = updateCheckMode == AUTO ? PATCHED_CACHE : VANILLA_CACHE;
+				// create the classloader for the jar while we hold the lock, and eagerly load and link all classes
+				// in the jar. Otherwise the jar can change on disk and can break future classloads.
+				classLoader = createJarClassLoader(jarFile);
+			}
 
 			SplashScreen.stage(.465, "Starting", "Starting Old School RuneScape");
 
-			Applet rs = loadClient(jar);
+			Applet rs = loadClient(classLoader);
 
 			SplashScreen.stage(.5, null, "Starting core classes");
 
@@ -424,12 +429,62 @@ public class ClientLoader implements Supplier<Applet>
 		}
 	}
 
-	private Applet loadClient(URL url) throws ClassNotFoundException, IllegalAccessException, InstantiationException
+	private ClassLoader createJarClassLoader(File jar) throws IOException, ClassNotFoundException
 	{
-		URLClassLoader rsClassLoader = new URLClassLoader(new URL[]{url}, ClientLoader.class.getClassLoader());
+		try (JarFile jarFile = new JarFile(jar))
+		{
+			ClassLoader classLoader = new ClassLoader(ClientLoader.class.getClassLoader())
+			{
+				@Override
+				protected Class<?> findClass(String name) throws ClassNotFoundException
+				{
+					String entryName = name.replace('.', '/').concat(".class");
+					JarEntry jarEntry = jarFile.getJarEntry(entryName);
+					if (jarEntry == null)
+					{
+						throw new ClassNotFoundException(name);
+					}
 
+					try
+					{
+						InputStream inputStream = jarFile.getInputStream(jarEntry);
+						if (inputStream == null)
+						{
+							throw new ClassNotFoundException(name);
+						}
+
+						byte[] bytes = ByteStreams.toByteArray(inputStream);
+						return defineClass(name, bytes, 0, bytes.length);
+					}
+					catch (IOException e)
+					{
+						throw new ClassNotFoundException(null, e);
+					}
+				}
+			};
+
+			// load all of the classes in this jar; after the jar is closed the classloader
+			// will no longer be able to look up classes
+			Enumeration<JarEntry> entries = jarFile.entries();
+			while (entries.hasMoreElements())
+			{
+				JarEntry jarEntry = entries.nextElement();
+				String name = jarEntry.getName();
+				if (name.endsWith(".class"))
+				{
+					name = name.substring(0, name.length() - 6);
+					classLoader.loadClass(name);
+				}
+			}
+
+			return classLoader;
+		}
+	}
+
+	private Applet loadClient(ClassLoader classLoader) throws ClassNotFoundException, IllegalAccessException, InstantiationException
+	{
 		String initialClass = config.getInitialClass();
-		Class<?> clientClass = rsClassLoader.loadClass(initialClass);
+		Class<?> clientClass = classLoader.loadClass(initialClass);
 
 		Applet rs = (Applet) clientClass.newInstance();
 		rs.setStub(new RSAppletStub(config));