/** Determine JDK level of an applet. */ private void findAppletJDKLevel(Applet applet) { // To determine the JDK level of an applet, the // most reliable way is to check the major version // of the applet class file. // synchronized on applet class object, so calling from // different instances of the same applet will be // serialized. Class<?> appletClass = applet.getClass(); synchronized (appletClass) { // Determine if the JDK level of an applet has been // checked before. Boolean jdk11Target = loader.isJDK11Target(appletClass); Boolean jdk12Target = loader.isJDK12Target(appletClass); // if applet JDK level has been checked before, retrieve // value and return. if (jdk11Target != null || jdk12Target != null) { jdk11Applet = (jdk11Target == null) ? false : jdk11Target.booleanValue(); jdk12Applet = (jdk12Target == null) ? false : jdk12Target.booleanValue(); return; } String name = appletClass.getName(); // first convert any '.' to '/' name = name.replace('.', '/'); // append .class final String resourceName = name + ".class"; byte[] classHeader = new byte[8]; try (InputStream is = AccessController.doPrivileged( (PrivilegedAction<InputStream>) () -> loader.getResourceAsStream(resourceName))) { // Read the first 8 bytes of the class file int byteRead = is.read(classHeader, 0, 8); // return if the header is not read in entirely // for some reasons. if (byteRead != 8) return; } catch (IOException e) { return; } // Check major version in class file header int major_version = readShort(classHeader, 6); // Major version in class file is as follows: // 45 - JDK 1.1 // 46 - JDK 1.2 // 47 - JDK 1.3 // 48 - JDK 1.4 // 49 - JDK 1.5 if (major_version < 46) jdk11Applet = true; else if (major_version == 46) jdk12Applet = true; // Store applet JDK level in AppContext for later lookup, // e.g. page switch. loader.setJDK11Target(appletClass, jdk11Applet); loader.setJDK12Target(appletClass, jdk12Applet); } }