diff --git a/build.gradle b/build.gradle
index 136ca79..42440d8 100644
--- a/build.gradle
+++ b/build.gradle
@@ -27,7 +27,10 @@ jar {
 			'Implementation-Timestamp': new Date().format("yyyy-MM-dd'T'HH:mm:ssZ"),
 			'Git-Commit': gitInfo.gitHashFull,
 			'Git-IsClean': gitInfo.isCleanTag,
-			'Premain-Class': 'org.to2mbn.authlibinjector.javaagent.AuthlibInjectorPremain'
+			'Premain-Class': 'org.to2mbn.authlibinjector.javaagent.AuthlibInjectorPremain',
+			'Agent-Class': 'org.to2mbn.authlibinjector.javaagent.AuthlibInjectorPremain',
+			'Can-Retransform-Classes': true,
+			'Can-Redefine-Classes': true
 		)
 	}
 }
diff --git a/src/main/java/org/to2mbn/authlibinjector/AuthlibInjector.java b/src/main/java/org/to2mbn/authlibinjector/AuthlibInjector.java
index 9fb32ec..02b9ba7 100644
--- a/src/main/java/org/to2mbn/authlibinjector/AuthlibInjector.java
+++ b/src/main/java/org/to2mbn/authlibinjector/AuthlibInjector.java
@@ -15,7 +15,7 @@ import org.to2mbn.authlibinjector.transform.YggdrasilKeyTransformUnit;
 
 public final class AuthlibInjector {
 
-	private static final String[] nonTransformablePackages = new String[] { "java.", "javax.", "com.sun.",
+	public static final String[] nonTransformablePackages = new String[] { "java.", "javax.", "com.sun.",
 			"com.oracle.", "jdk.", "sun.", "org.apache.", "com.google.", "oracle.", "com.oracle.", "com.paulscode.",
 			"io.netty.", "org.lwjgl.", "net.java.", "org.w3c.", "javassist.", "org.xml.", "org.jcp.", "paulscode.",
 			"com.ibm.", "joptsimple.", "org.to2mbn.authlibinjector." };
diff --git a/src/main/java/org/to2mbn/authlibinjector/javaagent/AuthlibInjectorPremain.java b/src/main/java/org/to2mbn/authlibinjector/javaagent/AuthlibInjectorPremain.java
index 3154223..7d64df5 100644
--- a/src/main/java/org/to2mbn/authlibinjector/javaagent/AuthlibInjectorPremain.java
+++ b/src/main/java/org/to2mbn/authlibinjector/javaagent/AuthlibInjectorPremain.java
@@ -1,21 +1,92 @@
 package org.to2mbn.authlibinjector.javaagent;
 
 import static org.to2mbn.authlibinjector.AuthlibInjector.bootstrap;
+import static org.to2mbn.authlibinjector.AuthlibInjector.debug;
 import static org.to2mbn.authlibinjector.AuthlibInjector.info;
+import static org.to2mbn.authlibinjector.AuthlibInjector.nonTransformablePackages;
 import java.lang.instrument.Instrumentation;
+import java.util.Arrays;
 
 public class AuthlibInjectorPremain {
 
 	public static void premain(String arg, Instrumentation instrumentation) {
 		try {
-			info("launched from javaagent");
-			if (arg != null && !arg.isEmpty()) {
-				System.setProperty("org.to2mbn.authlibinjector.config", arg);
-			}
-			bootstrap(instrumentation::addTransformer);
+			info("launched from premain");
+			initInjector(arg, instrumentation, false);
 		} catch (Throwable e) {
 			// prevent the exception being thrown to VM
 			e.printStackTrace();
 		}
 	}
+
+	public static void agentmain(String arg, Instrumentation instrumentation) {
+		try {
+			info("launched from agentmain");
+			initInjector(arg, instrumentation, true);
+		} catch (Throwable e) {
+			// prevent the exception being thrown to VM
+			e.printStackTrace();
+		}
+	}
+
+	public static void initInjector(String arg, Instrumentation instrumentation, boolean needsRetransform) {
+		setupConfig(arg);
+
+		boolean retransformSupported = instrumentation.isRetransformClassesSupported();
+		boolean retransformEnabled = retransformSupported && needsRetransform;
+		bootstrap(x -> instrumentation.addTransformer(x, retransformEnabled));
+
+		if (needsRetransform) {
+			if (retransformSupported) {
+				info("start retransforming");
+				doRetransform(instrumentation);
+			} else {
+				info("retransforming is not supported");
+			}
+		}
+	}
+
+	private static void setupConfig(String arg) {
+		if (arg != null && !arg.isEmpty()) {
+			System.setProperty("org.to2mbn.authlibinjector.config", arg);
+		}
+	}
+
+	private static void doRetransform(Instrumentation instrumentation) {
+		try {
+			long t0 = System.currentTimeMillis();
+			Class<?>[] classToRetransform = getClassesToRetransform(instrumentation);
+			if (classToRetransform.length > 0) {
+				instrumentation.retransformClasses(classToRetransform);
+			}
+			long t1 = System.currentTimeMillis();
+			info("retransforming finished in {0}ms", t1 - t0);
+		} catch (Throwable e) {
+			info("unable to retransform");
+			e.printStackTrace();
+		}
+	}
+
+	private static Class<?>[] getClassesToRetransform(Instrumentation instrumentation) {
+		Class<?>[] loadedClasses = instrumentation.getAllLoadedClasses();
+		Class<?>[] dest = new Class[loadedClasses.length];
+		int idx = 0;
+		for (Class<?> clazz : loadedClasses) {
+			if (instrumentation.isModifiableClass(clazz)) {
+				boolean toRetransform = true;
+				for (String nonTransformablePackage : nonTransformablePackages) {
+					if (clazz.getName().startsWith(nonTransformablePackage)) {
+						toRetransform = false;
+						break;
+					}
+				}
+				if (toRetransform) {
+					dest[idx++] = clazz;
+				}
+			}
+		}
+		debug("loaded {0} classes, {1} to retransform", loadedClasses.length, idx);
+		return Arrays.copyOf(dest, idx);
+	}
+
 }