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2.3 KiB
Markdown
86 lines
No EOL
2.3 KiB
Markdown
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title = "Generators"
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description = "A java 24 library that adds stackless coroutines to standard java code by transforming method bytecode into state machines at load time"
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date = 2025-05-07
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[taxonomies]
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tags = ["java", "bytecode"]
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category = ["project"]
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With the release of Java 24 came the official Class-File API giving a stable and (relatively) ergonomic interface for reading, modifying, and writing java classes/bytecode at runtime with no external dependencies. Seeing this I decided to play around with it and see what fun I could have.
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# Bytecode, Stack Machines, and Registers
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Java (and other JVM languages) are compiled to a set of [bytecode instructions](https://en.wikipedia.org/wiki/List_of_JVM_bytecode_instructions).
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These instructions primarily operate on a stack (fifo) using 0-n operands from the top and pushing 0-1 values back onto the stack.
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An example would be the `iadd` instruction which pops two integers off the stack and pushes the sum of them.
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A more complex example would be something like `invokevirtual <methodref_index>` which calls a method on a class.
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`methodref_index` is an index (in this case a 2 byte index) into the class files `constant_pool`[^1]
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[^1]: https://docs.oracle.com/javase/specs/jvms/se23/html/jvms-4.html#jvms-4.1
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## Stack manipulation
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Sometimes it's
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## Registers
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# Stack Machine and Registers
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# Decompilation
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# Interlude, what's a stack map?
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# Tracking everything everywhere
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# Putting everything together
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# Injection
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Take a look at the [GitHub](https://github.com/ParkerTenBroeck/generators)
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```java,linenos
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public static Future<Void, IOException> echo(
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@Cancellation("close") Socket socket
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) throws IOException {
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try(socket){
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var buffer = ByteBuffer.allocate(4096*2);
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while(true){
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bytes_received = socket.read(buffer).await() + bytes_received;
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buffer.flip();
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bytes_sent = socket.write_all(buffer).await() + bytes_sent;
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buffer.clear().limit(buffer.capacity());
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}
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}
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}
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```
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```java,linenos
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public static Gen<Long, Void> primes() {
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long number = 1;
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Gen.yield(2L);
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outer: while(true){
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number += 2;
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for(long i=2; i <= Math.sqrt(number); i ++){
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if(number%i==0)continue outer;
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}
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Gen.yield(number);
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}
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}
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``` |