A JVM stack is composed offrames, each pushed onto the stack when a method is invoked and popped from the stack when the method completes (either by returning normally or by throwing an exception). For example, there are no instructions that directly operate on boolean values.
There's a keyword new in Java but there's also a bytecode instruction called new. That means that the code compiled in a Linux machine and the code compiled in a Windows machine should work in the JVM either ways. If theodesp is not suspended, they can still re-publish their posts from their dashboard. In fact, a large portion of the instruction set is denoted to the arithmetic. Many IT professionals might not have had the time to goof around with assembler or machine code, soJava bytecodecan seem like an obscure piece of low-level magic. Type conversion happens for instance when we want to assign an integer value to a variable which type is long.
At last, we indicate that the generating bytecode for the method is complete with visitEnd() method. Next,astore_1 pops that Point reference and assigns it to the local variable at index 1 (the a in astore_1 indicates this is a reference value). We have touched the method invocation topic slightly in the class instantiation part: the method was invoked via invokespecial instruction which resulted in the constructor call. Well, here it is! Next, instructions iconst_1 and iconst_2 are used to load constants 1 and 2 to the stack and store them in LocalVariableTable slots 2 and 3 respectively by the instructions istore_2 and istore_3. The last step loads the references to the two Point objects from local variables at indexes 1 and 2 (using aload_1 and aload_2 respectively), and invokes the area method using invokevirtual, which handles dispatching the call to the appropriate method based on the actual type of the object. Heres what ASM code for main method looks like: By calling the visitMethod() again, we generated the new method definition with the name, modifiers and the signature. Basically, this is the super() call; The main method creates an instance of MovingAverage class and returns. You might have noticed that some of the instructions are referring to some numbered parameters with #1, #2, #3. Why three instructions instead of one, you ask? Java bytecode runs the JVM quietly in the background most of the time--so the average developer rarely needs to consider it. Opinions expressed by DZone contributors are their own. They expect operands of type, respectively, int, long, float, and double. A Java programmer, normally, does not need to be aware of how Java bytecode works. The best way to learn to use ASM is to write a Java source file that is equivalent to what you want to generate and then use the ASMifier mode of the Bytecode Outline plugin for Eclipse (or the ASMifier tool) to see the equivalent ASM code. Let's modify the example and introduce a class Point to encapsulate XY coordinates. We will start with a class that will serve as an entry point for our example application, the moving average calculator. Java bytecode is the machine code that enables the JVM to interpret and compile language code such as Java, Scala, Groovy, Kotlin and a dozen more in order to deliver applications to hungry consumers. To understand the details of the bytecode, we need to have an idea of the model of execution of the bytecode. Plus there's a lot of instructions that are used to convert between the types. And then we see a chunk of bytecode that calls Stream.close along with branches that call Throwable.addSuppressed. ACC_SUPER was introduced to correct a problem with the invocation of super methods with the invokespecial instruction. After a couple of months, I needed those changes in source form again (which took quite an effort to come up with), but I could not find them! Let's now add some more code into our initial example: We submit two numbers to the MovingAverage class and ask it to calculate the average of the current values. Luckily the compiled code still existed on that remote server. The bytecode obtained from this code is as follows: After creating the local variable of type MovingAverage the code stores the value in a local variable ma, with the astore_1 instruction: 1 is the slot number of ma in the LocalVariableTable. Built on Forem the open source software that powers DEV and other inclusive communities. Line 49 in our Bytecode is: You can continue reading the bytecode and understand what invokevirtual, getstatic or ifeq mean. We're a place where coders share, stay up-to-date and grow their careers. Why do we need to know about such low-level stuff in the first place? In other words, the calling method prepares all arguments of the to-be-called method by pushing them onto the operand stack in the correct order. It first loads the first integer argument onto the operand stack (iload_0). references to fields or methods) and constants known as the constant pool. The dup instruction duplicates the value on top of the stack. At the end of the loop it loop counter is incremented by 1 and the loop jumps back to the beginning to validate the loop condition again: As you have seen already, there's a number of instructions that perform all kind of arithmetics in Java bytecode. This is the basic code that gets generated by the compiler for a try-with-resources statement. Depending on the nature of the instructions, we can group these into several broader groups: There are also a number of instructions of more specialized tasks such as synchronization and exception throwing. You don't need to master the understanding of each instruction and the exact flow of execution to gain an idea about what the program does based on the bytecode at hand. At the end we call to visitMaxs() - this is to ask ASM to recompute the maximum stack size. Again, visitCode(), visitMaxs() and visitEnd() methods are used the same way as in case with the constructor.
Get started with a free 14-day trial! The other new information is obviously the code for the calc method itself. We can apply the -verbose argument to javap when disassembling the class: Heres some interesting parts that it prints: There's a bunch of technical information about the class file: when it was compiled, the MD5 checksum, which *.java file it was compiled from, which Java version it conforms to, etc. In the above example, there is only one method, the main method. A set of flags follow that describe the method as public (ACC_PUBLIC) and static (ACC_STATIC). This can be illustrated below. It has various features enabled based on the current version. Reading compiled Java bytecode can be tedious, even for experienced Java developers. Glad that others pick it up as well! The dup instruction is used to duplicate the value on the top of the stack. Want to see how much time JRebel can save you in application development? It is easy to evaluate such expression by using a stack: The result, 3, is on the top of the stack after the 'add' instruction executes. For example, in my case, I wanted to check if the code employed a Java stream to read a file, and whether the stream was properly closed. All local variables are referenced in the above instructions except the first one (at index 0), which holds the reference to the args argument. Here is the list for the OpenJDK 9 for example: Let's go a little bit more practical and try to understand and read bytecode from a simple program. The majority of bytecode has this characteristic of having different forms of the same functionality depending on the operand types. Memory for the object is allocated on the heap, and a reference to the object is pushed on the operand stack. We can copy the compiled .class files from linux to windows and run them there without issues and vice versa. There are some more complex instructions: swap, dup_x1 and dup2_x1, for instance. So we have completed the following operation: String strOSName = System.getProperty("os.name"); Conditional branch which means. Build a type safe React App with ReasonML, Part 1, 30 minute introduction to ReasonML for React Developers. It doesnt happen automagically, and this is why some bytecode instructions are generated into the default constructor. ASM exposes the internal aggregate components of a given Java class through its visitor oriented API. There are a few other things that reveal itself when using -verbose argument with javap. For instance, in the listing above, before calling the submit method, we have to load the value of the parameter to the stack again: After calling the getAvg() method the result of the execution locates on the top of the stack and to store it to the local variable again the dstore instruction is used since the target variable is of type double.
Java developers should be familiar with all of the above types, except returnAddress, which has no equivalent programming language type. This is why we need to duplicate the reference in advance so that after the constructor returns we can assign the object instance into a local variable or a field. But was is ACC_SUPER for? The compiled bytecode for the main method is shown below: The new instructions encountereted here are new , dup, and invokespecial. Let's explore more about those: In JVM, every constructor of a class, even if it's not defined, is invoked as a call to
This means that if the runtime wants to call method2, it will always find it at position 2. There are various memory components used by a JVM process, but only the JVM stacks need to be examined in detail to essentially be able to follow bytecode instructions: PC register: for each thread running in a Java program, a PC register stores the address of the current instruction. The next method is where the fields x and y will get initialized. The same procedure is repeated for creating and initializing the second Point instance, which is assigned to variable b. The operand stack is also used to receive return values from methods. The first one (pointA, which comes from variable a) is actually the instance on which the method is invoked (otherwise referred to as this in the programming language), and it will be passed in the first local variable of the new frame for the area method. If you want to learn more about the JVM ecosystem I can recommend the following resources: Templates let you quickly answer FAQs or store snippets for re-use. Stores reference from the top stack position into local frame at index 1. The example above is expressed with Java bytecode instructions is identical, and the only difference is that the opcodes have some specific semantics attached: The opcodes iconst_1 and iconst_2 put constants 1 and 2 to the stack. In the debugger, we can drop frames one by one, however the state of the fields will not be rolled back. Before learning about the bytecode instruction set though, let's get familiar with a few things about the JVM that are needed as a prerequisite. Create a new Java program using the editor dialog. See the original article here. Without Java bytecode behind the scenes, the JVM would not be able to compile and mirror the non-bytecode Java code developers write to add new features, fix bugs and produce beautiful apps. Here is some explanations for a few of them: Pushes item #5 from the constant pool which is "os.name" into the stack. ", JRebel by Perforce 2022 Perforce Software, Inc.Terms of Use |Privacy Policy| Data Processing Policy |Sitemap. Are you sure you want to hide this comment? Heap: memory shared by all threads and storing objects (class instances and arrays). For example, there are several add instructions to add two numbers: iadd, ladd, fadd, dadd. The resulting double replaces the top of the operand stack. To give you a very gentle introduction we will generate a Hello World example using the ASM library and add a loop to print the phrase an arbitrary number of times. For instance, i prefix stands for integer and therefore the iadd instruction indicates that the addition operation is performed for integers. Over 2 million developers have joined DZone. You can also find the denoted constant definitions in the constant pool: The constant definitions are composable, meaning the constant might be composed from other constants referenced from the same table. This is where everyone has his own approach it writing code with ASM. For further actions, you may consider blocking this person and/or reporting abuse. One more interesting thing to notice about the LocalVariableTable is that the first slot is occupied with the parameter(s) of the method. That said, if you create a new instance of the class, access a static field or call a static method, the static initializer is triggered. You can see that there are three variables in the LocalVariableTable that aren't really mentioned in the source code: arr$, len$, i$ - those are the loop variables. When working with the JVM ecosystem, it's important to spend some time and understand what is happening behind the scenes. The only instruction that doesn't require the value on the stack is the increment instruction, iinc, which operates on the value sitting in LocalVariableTable directly. Desperate times call for desperate measures. Certain opcode instructions push values onto the operand stack; others take operands from the stack, manipulate them, and push the result. The JVM in simple words is an engine that reads compiled code in a format that is specified from a Java Virtual Machine Specification and executes it on the current machine. ASM outline plugin in IntelliJ IDEA. To swap the two double values we would like to use the swap instruction but the problem is that it works only with one-word instructions, meaning it will not work with doubles, and swap2 instruction does not exist. class files) and it is executed inside the JVM. We will now change our example so that it will handle an arbitrary number of numbers that can be submitted to the MovingAverage class: Assume that the numbers variable is a static field in the same class. The store instruction will always remove the value from the top of the stack. and only accessible to Theofanis Despoudis. The reason: Some of the opcodes have parameters that take up space in the bytecode array. As the constructor call doesn't return a value, after calling the method on the object the object will be initialized but the stack will be empty so we wouldn't be able to do anything with the object after it was initialized. The instructions from address 0 to 8 will do the following: iconst_1: Push the integer constant 1 onto the operand stack. [3] : Understanding bytecode makes you a better programmer http://www.ibm.com/developerworks/ibm/library/it-haggar_bytecode. This will consume the top stack frame variable that we passed before. Java is statically typed, which affects the design of the bytecode instructions such that an instruction expects itself to operate on values of specific types. Here's an example: class A, with methods method1 and method2 and a subclass B, which derives method1, overrides method2, and declares new method3. Once unpublished, this post will become invisible to the public Put simply, Java bytecode is the intermediate representation of Java code (i.e. We see occurrences of java/util/stream/Stream where forEach is called, preceded by a call to InvokeDynamic with a reference to a Consumer. The API itself is not very broad - with a limited set of classes you can achieve pretty much all you need. You probably always knew that if you dont specify any constructor for a class theres still a default one, but maybe you didnt realize where it actually is. This is what makes writing such code rather a complicated task. Moreover, bytecode is simpler than native machine code because the JVM architecture is rather simple, hence simplifying the instruction set.
Why is that? In our program we have currently define one main method. The same procedure is applied to compute Math.pow(b, 2): The next instruction,dadd, pops the top two intermediate results, adds them, and pushes the sum back to the top. Every time a method is invoked a frame is created. Why is the reverse Polish notation any good here? The static initializer of the class isn't called directly, but triggered by one of the following instructions: new, getstatic, putstatic or invokestatic. But it is the form of the instructions that the JVM executes, so it is essential to the areas of tooling and program analysis, where the applications can modify the bytecode to adjust the behavior according to the application's domain. DEV Community A constructive and inclusive social network for software developers. Object deallocation is managed by a garbage collector. The above code just retrieves the os.name system property and checks if it contains the string linux. I hope you liked what we did today, and understood the basic blocks of JVM and Bytecodes. From a technical POV, Java bytecode is the code set used by the Java Virtual Machine that is JIT-compiled into native code at runtime. The variable arr$ stores the reference value of the numbers field from which the length of the loop, len$, is derived using the arraylength instruction. Let's go though one by one: Interfaces+Fields: This section displays any interface and field declarations. It will become hidden in your post, but will still be visible via the comment's permalink. We just proved that the default constructor actually exists in the compiled class, so it is java compiler who generates it. As a result, the two doubles will be swapped. For instance, with invokestatic we know exactly which method to call: it is static, it belongs to only one class. High school student. The caveat is that double takes two slots in the stack, which means that if we have two double values on the stack they occupy four slots. While the stack is used for execution, local variables are used to save the intermediate results and are in direct interaction with the stack. Each thread has a JVM stack which stores frames. The above instructions are the minimum required to call
Hence, the next instruction is usually one of the following: While a call to is a constructor invocation, there's another similar method, which is invoked even earlier.