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Java Applets ​

Status: Planned — No implementation started.

Format ​

Java Applets are distributed as .jar files (ZIP archives containing .class files) or as individual .class files. The format is:

  • JVM bytecode — standardized, exhaustively documented in the JVM specification
  • Class files — magic 0xCAFEBABE, constant pool, access flags, field/method descriptors
  • Jar manifest — META-INF/MANIFEST.MF specifying the main class and applet class

Java bytecode is one of the best-documented and most-reversible formats in existence. Mature open-source decompilers (CFR, Fernflower, Procyon) can recover Java source that closely matches the original.

Lifting Strategy ​

Full recompilation (Tier 2).

Java bytecode → IR is highly practical:

  • JVM bytecode has a clean type system (all types annotated in the constant pool and descriptors)
  • The class hierarchy maps directly to TypeScript/Rust class hierarchies
  • Generics information is available in class signatures (Java 5+)
  • Method signatures are fully typed
  1. Unpack the .jar and parse .class files
  2. Decode JVM bytecode per method using the JVM specification
  3. Emit IR with full type information from class/method descriptors
  4. Identify java.applet.*, java.awt.*, and javax.swing.* API boundaries

What Needs Building ​

Format Parser (new crate: reincarnate-frontend-java) ​

  • [ ] JAR/ZIP extractor
  • [ ] Class file parser:
    • [ ] Constant pool (UTF8, Class, FieldRef, MethodRef, InterfaceMethodRef, String, Integer, Float, Long, Double, NameAndType, MethodHandle, MethodType, Dynamic, InvokeDynamic, Module, Package)
    • [ ] Field descriptors (type signatures)
    • [ ] Method descriptors (parameter + return types)
    • [ ] JVM bytecode decoder — ~200 opcodes, well-specified
    • [ ] Stack map frames (Java 6+ for type verification)
    • [ ] Exception table
    • [ ] Annotations (RuntimeVisibleAnnotations, RuntimeInvisibleAnnotations)
    • [ ] Generics signature (Signature attribute)

JVM Instruction Set Categories ​

  • Load/store: iload, aload, istore, astore, array variants, field access (getfield, putfield, getstatic, putstatic)
  • Arithmetic: iadd, isub, imul, idiv, irem, etc. for int/long/float/double
  • Conversions: i2l, l2i, i2f, f2i, i2b, i2c, i2s, etc.
  • Comparison: if_icmpeq, if_acmpeq, ifnull, ifnonnull, lcmp, fcmpl, dcmpg, etc.
  • Control: goto, jsr/ret (subroutines, Java <6), tableswitch, lookupswitch, return variants
  • Invocation: invokevirtual, invokeinterface, invokestatic, invokespecial, invokedynamic (Java 7+)
  • Object: new, newarray, anewarray, multianewarray, instanceof, checkcast
  • Exception: athrow, monitorenter, monitorexit

IR Mapping ​

  • Java method → IR function
  • Java instanceof / checkcast → IR type check / cast operations
  • try/catch/finally → IR exception handlers (blocked on IR try/catch support)
  • invokevirtual → IR Call on self with virtual dispatch
  • invokestatic → IR direct Call
  • invokedynamic → IR indirect call (lambda desugaring — Java 8+)
  • synchronized → no-op in single-threaded web environment

Replacement Runtime (runtime/java-applets/ts/) ​

Java Applets target java.applet.Applet which is a subclass of java.awt.Panel. The AWT/Swing API surface needed:

Core AWT (required for applets):

  • [ ] Applet — lifecycle: init(), start(), stop(), destroy(), paint(Graphics)
  • [ ] Graphics / Graphics2D — drawing primitives
  • [ ] Component — paint, repaint, event handling, preferred size
  • [ ] Container — add/remove components, layout managers
  • [ ] Panel, Frame, Canvas
  • [ ] Layout managers: FlowLayout, BorderLayout, GridLayout
  • [ ] Event model: ActionListener, MouseListener, KeyListener, FocusListener
  • [ ] Font, FontMetrics
  • [ ] Color, Image, MediaTracker
  • [ ] AudioClip

Common Swing (for Swing applets):

  • [ ] JPanel, JLabel, JButton, JTextField, JTextArea, JComboBox, JList
  • [ ] JScrollPane, JTabbedPane, JSplitPane
  • [ ] JDialog, JOptionPane
  • [ ] SwingUtilities.invokeLater → setTimeout(fn, 0)
  • [ ] Swing look-and-feel (Metal L&F is default; approximate with CSS)

Java standard library stubs:

  • [ ] java.lang — String, StringBuilder, Math, Integer, Double, etc.
  • [ ] java.util — ArrayList, HashMap, Collections, Arrays, Random, Date, Calendar
  • [ ] java.io — streams, readers, writers (mapped to fetch/Blob/ArrayBuffer)
  • [ ] java.net.URL — openStream() → fetch

Known Challenges ​

  • Threading — Java applets often use threads (Thread, Runnable, synchronized). JavaScript is single-threaded. Lightweight threading (cooperative coroutines via state machines) can handle simple cases; heavy multi-threading requires Web Workers.
  • AWT/Swing size — The full AWT+Swing API is enormous. Most applets use a small subset; coverage can grow organically from test cases.
  • invokedynamic (Java 8+) — Lambda expressions and method references use invokedynamic with BootstrapMethods. These need special handling to recover the original lambda.
  • Reflection — Class.forName(), getDeclaredMethod(), etc. break static analysis.
  • Serialization — java.io.Serializable applets that persist state require a binary serialization format.

References ​