String concatenated with a null reference might get you such a message: Object obj = null; throw new ArrayIndexOutOfBoundsException(“” + obj); If you’re using an Oracle JVM you may want to add -XX:-OmitStackTraceInFastThrow as an additional parameter to see if it helps. For some basic exceptions, the JVM reuses the same exception instance after a … Read more
There are several reasons. First off, it’s not necessary for performance. The JVM already optimizes things at runtime, so there’s no point in adding redundant complexity to the compiler. However, another major reason noone has mentioned here is debugging. Making the bytecode as close to the original source as possible makes it a lot easier … Read more
Here is a working “hello world” made with ObjectWeb ASM (a library which I recommend): package hello; import java.lang.reflect.Method; import org.objectweb.asm.ClassWriter; import org.objectweb.asm.Label; import org.objectweb.asm.MethodVisitor; import org.objectweb.asm.Opcodes; public class HelloWorldASM implements Opcodes { public static byte[] compile(String name) { ClassWriter cw = new ClassWriter(0); MethodVisitor mv; cw.visit(V1_6, ACC_PUBLIC + ACC_SUPER, “hello/HelloWorld”, null, “java/lang/Object”, null); cw.visitSource(“HelloWorld.java”, … Read more
Previous Scala versions emitted version 49.0 of the byte code, which corresponded to Java 5. With Scala 2.10 version the default was changed to version 50.0, corresponding to Java 6 and which has the principal advantage of activating the faster verifier introduced with that version, so it should lead to (slightly) better run time performance. … Read more
A common application is to generate proxy classes at runtime, i.e. to create a subclass at runtime that intercepts all method invocations. Examples: Hibernate uses Proxies to intercept method invocations on entities to implement lazy loading, i.e. fetching the object from the database when it is first accessed. The Spring Framework uses Proxies to implement … Read more
The answer depends on what you mean by binary code. Java bytecode is a binary data format that includes loading information and execution instructions for the Java virtual machine. In that sense, Java bytecode is a special kind of binary code. When you use the term “binary code” to mean machine instructions for a real … Read more
I use reJ for editing class files directly. It allows you to edit instructions, methods, constant pool, diff classes, and a split view with a hex editor. It’s ridiculously awesome.
A good reference for Java bytecode specification is the The Java Virtual Machine Specification. See Chapter 4. The class File Format and Chapter 6. The Java Virtual Machine Instruction Set.
Yes, calling getClass() has become a canonical “test for null” idiom, as getClass() is expected to be a cheap intrinsic operation and, I suppose, HotSpot might be capable of detecting this pattern and reduce the operation to an intrinsic null-check operation, if the result of getClass() is not used. Another example is creating an inner … Read more