examples.md

JASM Examples

This is a collection of small examples in JASM. If there's an example you'd like to see here feel free to raise an issue or (even better!) submit a PR.

Some of these omit the class boilerplate for clarity - if you want to compile them you'll need to put them in a JASM class.

You can also fine lots of examples in the tests showing the syntax for the different instructions and with examples of how they're used.

There is also some more in-depth documentation and a few "standard recipes" in the cookbook.

Readable types

There's a smidge of syntactic sugar around types in case you just can't get used to the JVM internal names (for primitives), so you can do e.g. :

public myGreatMethod(int, long, java/util/List) java/util/List {
    
    // cool assembly stuff...
        
}

General Syntax

public class com/example/MyClass
extends com/example/Superclass
implements com/example/SomeInterface {
    public <init>()V {
        aload 0
        invokespecial com/example/Superclass.<init>()V
        return
    }

    public dontAddCanary(java/util/List)V {
        goto skipAdd

        aload 1
        ldc "CANARY"
        invokeinterface java/util/List.add(java/lang/Object)Z 
        
    skipAdd:
        return
    }
}

Loading Constants (ldc)

public ldcExample()V {
    ldc true                    // Syntactic sugar; Translates to integer 1
    ldc false                   // Syntactic sugar; Translates to integer 0
    ldc 10                      // int
    ldc 1000L                   // long
    ldc 5.0                     // float
    ldc 3.142d                  // double
    ldc "Test string"           // String
    ldc java/util/List          // Class
    ldc (java/util/String)I     // MethodType
    
    // Method handle
    ldc invokevirtual com/roscopeco/jasm/Tests.example()V
    
    // Dynamic constant
    ldc constdynamic DYNAMIC_CONST java/lang/Object {
        invokeinterface com/example/Bootstrap.constsite(java/lang/String)java/lang/invoke/CallSite
        ["Something else"]
    }
}

Exception Handling

With try/catch

public tryCatchExample(java/lang/Exception)java/lang/String {
    ldc "Fail"
    astore 2

    try {
        aload 1
        athrow
    } catch (java/io/IOException) {
        ldc "IOE"
        astore 2
    } catch (java/lang/NullPointerException) {
        ldc "NPE"
        astore 2
    } catch (java/lang/Exception) {
        ldc "EXCEPTION"
        astore 2
    }

    aload 2
    areturn
}

Manual exception handling

public manualExceptionHandlerTest()java/lang/Exception {
    exception tryBegin, tryEnd, catchBegin, java/lang/Exception  // Can be anywhere in the method...

  tryBegin:
    new java/lang/Exception
    dup
    ldc "Pass"
    invokespecial java/lang/Exception.<init>(java/lang/String)V
    athrow
  tryEnd:

    new java/lang/Exception
    dup
    ldc "Fail"
    invokespecial java/lang/Exception.<init>(java/lang/String)V
    areturn

  catchBegin:
    checkcast java/lang/Exception
    areturn
}

Annotations

@java/lang/Deprecated
@com/roscopeco/jasm/model/annotations/TestAnnotation(stringArg = "Yolo", classArg = java/util/List, arrayArg = { "one", "two" }, enumArg = [com/roscopeco/jasm/model/annotations/TestEnum.THREE])
class ComplexAnnotatedClass {
    @java/lang/Deprecated(since = "2002")
    public test(@com/roscopeco/jasm/model/annotations/TestAnnotation I, @java/lang/Deprecated(since = "3003") @com/roscopeco/jasm/model/annotations/TestAnnotation(classArg = java/util/List) java/lang/String)V {
        
    }
}

N.B. Enum args must be enclosed in square brackets, while curly brackets are used for arrays.

Invokedynamic

public doBasicInvokeDynamicTest()java/lang/String {
    invokedynamic get()java/util/function/Supplier {
        invokestatic java/lang/invoke/LambdaMetafactory.metafactory(
            java/lang/invoke/MethodHandles$Lookup,
            java/lang/String,
            java/lang/invoke/MethodType,
            java/lang/invoke/MethodType,
            java/lang/invoke/MethodHandle,
            java/lang/invoke/MethodType,
        )java/lang/invoke/CallSite
        [
            ()java/lang/Object,
            invokestatic com/roscopeco/jasm/model/TestBootstrap.lambdaGetImpl()java/lang/String,
            ()java/lang/String
        ]
    }
    
    invokeinterface java/util/function/Supplier.get()java/lang/Object
    checkcast java/lang/String
    areturn
}

Dynamic constants (constdynamic)

public testLdcDynamicConst()java/lang/String {
    ldc constdynamic DYNAMIC_CONST_FOR_TEST java/lang/String {
        invokestatic java/lang/invoke/ConstantBootstraps.getStaticFinal(
            java/lang/invoke/MethodHandles$Lookup,
            java/lang/String,
            java/lang/Class,
            java/lang/Class,
        )java/lang/Object
        [com/roscopeco/jasm/model/TestBootstrap]
    }

    areturn
}