Creating a java compiler plugin

Testing the Plugin

We need to be able to test our plugin. It involves the following:

• compile the test source
• run the compiled binaries and ensure that they behave as expected

For this, we need to introduce a few auxiliary classes.

SimpleSourceFile exposes the given source file’s text to the Javac:

SimpleClassFile holds the compilation result as a byte array:

SimpleFileManager ensures the compiler uses our bytecode holder:

Finally, all of that is bound to the in-memory compilation:

After that, we need only to run the binaries:

A test might look like this:

Here we’re compiling a Test class with a service() method that has a parameter annotated with @Positive. Then, we’re running the Test class by setting a double value of -1 for the method parameter.

As a result of running the compiler with our plugin, the test will throw an IllegalArgumentException for the negative parameter.

Online Java Compilers

#1) OnlinedGdb

Price: FreePlatform Support: Windows

Online compiler and debugger tool for various languages including C/C++, Java, etc. It has an embedded gdb debugger.

Features:

• Supports various languages including C/C++, Java, Python, C#, VB, etc.
• First online IDE that gives debugging facility with embedded gdb debugger.
• Allows specifying command-line arguments.

Website: OnlinedGdb

#2) Jdoodle

Price: FreePlatform Support: Windows

Jdoodle is an online compiler developed with the aim of helping students to learn programming language. It is an online tool to compile and execute programs in Java, C/C++, PHP, Perl, Python, Ruby, HTML and many more.

Features:

• It provides a quick and easy way to compile and execute a few lines of code online.
• It provides a feature to save and share programs.
• Supports almost all the Java libraries.

Website: Jdoodle

#3) Codechef

Price: FreePlatform Support: Windows

This online IDE support multiple languages such as Java, C, C++, Python, and Ruby, etc. Suitable for various levels of programming and also contains a lot of tutorials using which a programmer can improve his/her skills.

Features:

• Supports multiple languages.
• Includes various difficulty levels for programming practice like a beginner, medium, hard, etc.
• Can open already existing programs in this editor.
• Has solid community support for programmers.

Website: Codechef

#4) Repl

Price: FreePlatform Support: Windows

A typical Repl online IDE will look as shown below:

Repl is the Powerful and simple online compiler, IDE, and interpreter that is capable of developing programs in 50+ languages including Java, Python, C, C++, JavaScript, etc.

Features:

• Interactive and Open-source IDE.
• IDE is cloud-based.
• Has powerful tools to learn and teach programming languages.
• We can share the code.

Website: Repl

#5) CompileJava

Price: FreePlatform Support: Windows

This is a fast and functional online Java compiler that always has the latest version of Java.

Features:

• Multiple themes that ensure ease of coding.
• Support for Optional command-line arguments.
• Multiple public classes are split automatically to files.
• Provides Applet support, including JPanel.
• The submissions made by the programmer are deleted within 5 minutes of execution (to accommodate applets) and are not stored for any other purpose.

Website: CompileJava

Setup

First, we need to add JDK’s tools.jar as a dependency for our project:

Every compiler extension is a class which implements com.sun.source.util.Plugin interface. Let’s create it in our example:

Let’s create it in our example:

For now, we’re just printing “Hello” to ensure that our code is successfully picked up and included in the compilation.

Our end goal will be to create a plugin that adds runtime checks for every numeric argument marked with a given annotation, and throw an exception if the argument doesn’t match a condition.

There’s one more necessary step to make the extension discoverable by Javac: it should be exposed through the ServiceLoader framework.

To achieve this, we need to create a file named com.sun.source.util.Plugin with content which is our plugin’s fully qualified class name (com.baeldung.javac.SampleJavacPlugin) and place it in the META-INF/services directory.

After that, we can call Javac with the -Xplugin:MyPlugin switch:

Note that we must always use a String returned from the plugin’s getName() method as a -Xplugin option value.

Plugin Lifecycle

A plugin is called by the compiler only once, through the init() method.

To be notified of subsequent events, we have to register a callback. These arrive before and after every processing stage per source file:

• PARSE – builds an Abstract Syntax Tree (AST)
• ENTER – source code imports are resolved
• ANALYZE – parser output (an AST) is analyzed for errors
• GENERATE – generating binaries for the target source file

There are two more event kinds – ANNOTATION_PROCESSING and ANNOTATION_PROCESSING_ROUND but we’re not interested in them here.

For example, when we want to enhance compilation by adding some checks based on source code info, it’s reasonable to do that at the PARSE finished event handler:

Extract AST Data

We can get an AST generated by the Java compiler through the TaskEvent.getCompilationUnit(). Its details can be examined through the TreeVisitor interface.

Note that only a Tree element, for which the accept() method is called, dispatches events to the given visitor.

For example, when we execute ClassTree.accept(visitor), only visitClass() is triggered; we can’t expect that, say, visitMethod() is also activated for every method in the given class.

We can use TreeScanner to overcome the problem:

In this example, it’s necessary to call super.visitXxx(node, value) to recursively process the current node’s children.

Модуляризация проекта

Пока что ничего нового, но давайте начнем модуляризацию нашего проекта. Для этого создадим модульный дескриптор (всегда называется и размещается в корневой директории ):

Команда для компиляции модуля в Java 9 отличается от того, что мы видели раньше. Использование старой команды с добавлением модуля к списку файлов приводит к ошибке:

Чтобы понять, почему наш код не компилируется, необходимо понять, что такое безымянные модули.

Любой класс, который загружается не из именованного модуля, автоматически выполняет часть безымянного модуля. В примере выше перед созданием модульного дескриптора наш код не был частью какого-либо модуля, следовательно, он был ассоциирован с безымянным модулем. Безымянный модуль — это механизм совместимости. Проще говоря, это позволяет разработчику использовать в приложениях Java 9 код, который не был модуляризирован. По этой причине код, относящийся к безымянному модулю, имеет правила сродни Java 8 и ранее: он может видеть все пакеты, экспортируемые из других модулей, и все пакеты безымянного модуля.

Когда модульный дескриптор добавляется к модулю, его код больше не является частью безымянного модуля и не может видеть код других модулей, пока не импортирует их. В случае выше модуль не требует никаких модулей, поэтому модуль библиотеки Greeting для него не виден. Он может видеть только пакеты модуля .

Модули в Java 9, за исключением неуловимого безымянного модуля описанного выше, должны объявлять, какие другие модули им необходимы. В случае с модулем  единственным требованием является библиотека Greeting. Но, как вы могли догадаться, эта библиотека (как и другие библиотеки, не поддерживающие Java 9) не является модулем Java 9. Как же нам включить её в проект?

В таком случае вам нужно знать имя jar-файла. Если у вас есть зависимость от библиотеки, которая не была конвертирована в модуль Java 9, вам надо знать, какой jar-файл вызывается для этой библиотеки, потому что Java 9 переведёт имя файла в валидный модуль.

Это называется автоматический модуль.

Так же, как и безымянные модули, автоматические модули могут читать из других модулей, и все их пакеты являются экспортируемыми. Но, в отличие от безымянных модулей, на автоматические можно ссылаться из явных модулей.

Чтобы узнать имя автоматического модуля, компилятор конвертирует неальфанумерические, поэтому что-то вроде превратится в имя модуля .

У нас есть библиотека с именем . Давайте переименуем jar-файл в :

Это более стандартное имя файла, и теперь мы можем сказать Java включить автоматический модуль с приемлемым именем . И можем вызывать его из модуля:

Модули не добавлены в . Как и обычные jar-файлы, они используют новый флаг . Теперь мы можем скомпилировать наши модули следующей командой:

Чтобы запустить командой мы можем использовать новый флаг , который принимает либо имя модуля, либо шаблон :

И мы получим вывод .

Для создания и использования в качестве исполняемого jar-файла выполните следующие команды:

Следующим шагом будет модуляризация библиотек, которые используются нашим приложением.

PROGRAMMATIC INTERFACE

javac supports the new Java Compiler API defined by the classes and interfaces in the package.

Example

To perform a compilation using arguments as you would give on the command line, you can use the following:

JavaCompiler javac = ToolProvider.getSystemJavaCompiler();
int rc = javac.run(null, null, null, args);

This will write any diagnostics to the standard output stream, and return the exit code that javac would give when invoked from the command line.

You can use other methods on the interface to handle diagnostics, control where files are read from and written to, and so on.

Old Interface

Note: This API is retained for backwards compatibility only; all new code should use the Java Compiler API, described above.

The class provides two static methods to invoke the compiler from a program:

public static int compile(String[] args);
public static int compile(String[] args, PrintWriter out);

The parameter represents any of the command line arguments that would normally be passed to the javac program and are outlined in the above section.

The parameter indicates where the compiler’s diagnostic output is directed.

The return value is equivalent to the exit value from javac.

Note that all other classes and methods found in a package whose name starts with (informally known as sub-packages of ) are strictly internal and subject to change at any time.

Типичные ошибки компиляции и запуска Java программ

Команда javac не найдена

Если при запуске javac, т.е. при попытке компиляции Java программы вы получаете ошибку:

"javac" не является внутренней или внешней
командой, исполняемой программой или пакетным файлом.

Это означает, что JDK не установлен. Либо установлен, но не настроены переменные окружения. Способы исправления очевидны:

• установить JDK
• настроить переменные окружения

Если JDK установлен, то можно обойтись без добавления переменной окружения. Для этого используйте абсолютный путь до исполнимого файла javac:

C:\Program Files\Java\jdk1.8.0_131\bin\javac.exe программа_для_компиляции.java

Ошибка Class names are only accepted if annotation processing is explicitly requested

Если попытаться скомпилировать программу следующим образом:

"C:\Program Files\Java\jdk1.8.0_131\bin\javac.exe" Welcome2

то возникнет ошибка:

error: Class names, 'Welcome', are only accepted if annotation processing is explicitly requested
1 error

Причина ошибки в том – что вы забыли указать расширение файла .java.

Ошибка записи (error while writing)

Запуск компиляции:

C:\Users\Alex>"C:\Program Files\Java\jdk1.8.0_131\bin\javac.exe" C:\Welcome.java

Компиляция заканчивается ошибкой:

C:\Welcome.java:1: error: error while writing Welcome: C:\Welcome.class (╬Єърчрэю т фюёЄєях)
public class Welcome {
       ^
1 error

Причина ошибки в том, что у компилятора (javac) недостаточно прав на запись в тот каталог, куда он пытается сохранить новый файл .class. Чтобы ошибка исчезла: предоставьте компилятору дополнительные права (запустите от имени администратора), либо сохраняйте в папку, на которую у текущего пользователя имеются права записи.

Ошибка «class is public, should be declared in a file named»

Запуск компиляции

"C:\Program Files\Java\jdk1.8.0_131\bin\javac.exe" C:\Welcome.java

который заканчивается примерной такой ошибкой

C:\Welcome.java:1: error: class Welcomee is public, should be declared in a file named Welcomee.java
public class Welcomee {
       ^
1 error

означает, что вы неправильно назвали класс в исходном коде программы. Имя класса должно совпадать с именем файла. В данном случае файл называется Welcome.java, а класс внутри программы назван Welcomee

Error: Could not find or load main class

Если попытаться запустить программу следующим образом:

java Welcome.class

то возникнет ошибка

Error: Could not find or load main class Welcome.class

Причина её в том, что не нужно было добавлять к названию файла расширение .class. Виртуальная машина автоматически добавляет расширение и в приведённом примере она ищет файл Welcome.class.class

Ошибка Error: Could not find or load main class при запуске Java программы по абсолютному пути

Эта ошибка возможно при запуске Java программы по абсолютному пути:

java C:\Welcome

Будет выведено:

Error: Could not find or load main class C:\Welcome

Ошибка возникает как в Windows, так и в Linux:

java /home/mial/Welcome
Error: Could not find or load main class .home.mial.Welcome

Если в терминале вы находитесь в той же директории, что и файл, который вы запускаете, то не нужно указывать абсолютный путь. Например, нужно запускать так:

cd C:\
java Welcome

Если же вы находитесь в другой директории, то нужно использовать опцию -cp, после которой указать путь до каталога, где размещена запускаемая программа. А далее указать запускаемый файл без расширения .class:

java -cp C:\ Welcome

Как видно из скриншота, командная строка находится в папке C:\WINDOWS\system32. Файл, который нам нужно запустить, находится в папке C:\ (корень диска). Мы указываем после ключа -cp папку C:\, а затем пишем имя файла программы без расширения – Welcome.

Аналогично нужно поступать в Linux. Пример команды:

java -cp /home/mial/ Welcome

Ошибка Main method not found in class

Если при запуске вы столкнулись с ошибкой:

Error: Main method not found in class Welcome, please define the main method as:
   public static void main(String[] args)
or a JavaFX application class must extend javafx.application.Application

Это означает, что вы не указали метод main, либо написали слово неправильно (например, Main вместо main).

JDoodle

JDoodle is an easy and quick way to compile and run source code in many programming languages including Java. Even more, JDoodle provides a terminal to work with MySql and MongoDB, peer programming tools, an API to execute code remotely, and an embedding feature for a blog or website.

Its UI has a user-friendly editor and input fields including:

• Command line arguments
• Interactive mode
• JDK versions 8 to 10 are available
• Attaching external libraries directly from the Maven repository. All we need is to specify the group, artifact ids and the version of the library.

Additionally, we can open, save, and download the source file.

Furthermore, the tool displays the compilation and execution along with CPU time in seconds. The memory usage is shown in kilobytes.

Important to note, that JDoodle allows having only one file. So it isn’t the best choice for multiple package projects or learning programming concepts like encapsulation and inheritance.

DESCRIPTION

The javac tool reads class and interface definitions, written in the Java programming language, and compiles them into bytecode class files. It can also process annotations in Java source files and classes.

There are two ways to pass source code file names to javac:

• For a small number of source files, simply list the file names on the command line.
• For a large number of source files, list the file names in a file, separated by blanks or line breaks. Then use the list file name on the javac command line, preceded by an @ character.

Source code file names must have suffixes, class file names must have suffixes, and both source and class files must have root names that identify the class. For example, a class called would be written in a source file called and compiled into a bytecode class file called .

Inner class definitions produce additional class files. These class files have names combining the inner and outer class names, such as .

You should arrange source files in a directory tree that reflects their package tree. For example, if you keep all your source files in C:\workspace, the source code for should be in C:\workspace\com\mysoft\mypack\MyClass.java.

By default, the compiler puts each class file in the same directory as its source file. You can specify a separate destination directory with -d (see , below).

Repl.it

Repl.it has an interactive programming environment supporting many languages. Also, we can create a simple web project including HTML and JS files to practice basic web skills. Its very popular among teachers, interviewers, universities, and colleges.

First of all, we need to sign up on the platform. It is worth mentioning that this requirement doesn’t exist in any of the previously listed compilers:

One important feature is a built-in version control which allows to save the current work, work with the session later, or compare the changes with each version.

Additionally, it has a multi-functional terminal emulator, provides with the rich API and code sharing/collaboration tools, third-party library support.

COMMAND LINE ARGUMENT FILES

To shorten or simplify the javac command line, you can specify one or more files that themselves contain arguments to the command (except options). This enables you to create javac commands of any length on any operating system.

An argument file can include javac options and source filenames in any combination. The arguments within a file can be space-separated or newline-separated. If a filename contains embedded spaces, put the whole filename in double quotes, and double each backslash ().

Filenames within an argument file are relative to the current directory, not the location of the argument file. Wildcards (*) are not allowed in these lists (such as for specifying ). Use of the ‘@‘ character to recursively interpret files is not supported. The options are not supported because they are passed to the launcher, which does not support argument files.

When executing javac, pass in the path and name of each argument file with the ‘@‘ leading character. When javac encounters an argument beginning with the character `@‘, it expands the contents of that file into the argument list.

Example — Single Arg File

You could use a single argument file named «» to hold all javac arguments:

C:\>javac @argfile

This argument file could contain the contents of both files shown in the next example.

Example — Two Arg Files

You can create two argument files — one for the javac options and the other for the source filenames: (Notice the following lists have no line-continuation characters.)

Create a file named «» containing:

     -d classes
     -g
     -sourcepath C:\java\pubs\ws\1.3\src\share\classes

Create a file named «» containing:

     MyClass1.java
     MyClass2.java
     MyClass3.java

You would then run javac with:

C:\>javac @options @classes

Example — Arg Files with Paths

The argument files can have paths, but any filenames inside the files are relative to the current working directory (not or ):

C:\>javac @path1\options @path2\classes

Introduction To Java IDE

Java is one of the popular and powerful programming languages as well as a platform. It is a high level and secured programming language that is used across several platforms in the world like Web applications, Android, Big Data, Banking Domain, Information Technology, Financial Services, etc.

To implement Java programming language we need certain environments where the user can develop codes and applications. Here comes the role of Java Integrated Development Environment (Java IDE). The need for Java IDE was felt as developers were facing issues while coding a huge application.

Huge applications will have a lot of classes & files, and thus, it gets difficult to debug them. With IDE, proper project management can be maintained. It provides hints on code completion, syntax errors, etc.

The Integrated Development Environment (IDE) is a software application that provides developers a platform with many features & facilities to develop Computer applications, Web pages, Tools, Services, etc.

The IDE tool will include text editors, debuggers, compilers, some features, and tools that will help in automation, testing and analyzing of an application development flow.

In simple terms, IDE allows developers to convert their logical code into some useful software applications.

Working Principle Of IDE

IDE follows a simple working principle that allows the developers to write logical code in its environment editor. Its compiler feature tells where all the errors are. The debug feature helps to debug the complete code and rectify errors.

Lastly, it helps in automating some parts and also assists to build a whole new software application. It is capable of supporting Model-Driven Development as well.

Core Functions Of IDE

• IDE should possess code completion capability for identifying the Java language functions and keyword.
• It should have strong resource management that helps to identify missing resources, headers, libraries, etc.
• A good debugging tool to test the developed application completely.
• Compile and build features.

Advantages:

• IDE takes very minimal time and effort as the entire concept of IDE is to make development easier and faster.
• It follows certain company standards, hence the working principle will be the same throughout and helps the coders.
• It comes with good project management tools and documents to automate many things.
• Useful in simplifying the development of database applications.
• It has features to develop a good user interface with text boxes, buttons, etc.

Disadvantages:

• IDE comes with a complex learning curve, thus having some expertise on these tolls will not be easy.
• It is not capable to remove bad code, design, and errors on its own. So the developer needs to be very careful while coding.
• It requires more memory as it uses the graphical user interface.
• It also has a restriction to interact with the database directly.

How To Select A Java IDE

Deciding which IDE or editor suits our needs depends on various factors including the nature of the projects or applications being developed, the process used by the development team, individual-level and skills as a programmer as well as the role in the organization.

Personal preferences and standardization of tools also play an important part in the selection of IDE or editor.

The major advantage of using an IDE for development is when a compiler is integrated with IDE, we get the entire package at one place so that we can complete the code, compile, debug, and execute the program in the same software.

IDEs have an attractive user interface and come packaged with all the elements of software development that we can use to develop software applications.

In this tutorial, we will discuss some of the IDE used for Java development along with the Compilers/IDEs that we can use for Java programming. For developing server-side Java applications, we use three IDEs often i.e. IntelliJ IDEA, Eclipse, and NetBeans.

We will review these three IDEs along with a few other popular ones.

ANNOTATION PROCESSING

javac provides direct support for annotation processing, superseding the need for the separate annotation processing tool, apt.

The API for annotation processors is defined in the and packages and subpackages.

Overview of annotation processing

Unless annotation processing is disabled with the -proc:none option, the compiler searches for any annotation processors that are available. The search path can be specified with the -processorpath option; if it is not given, the user class path is used. Processors are located by means of service provider-configuration files named on the search path. Such files should contain the names of any annotation processors to be used, listed one per line. Alternatively, processors can be specified explicitly, using the -processor option.

After scanning the source files and classes on the command line to determine what annotations are present, the compiler queries the processors to determine what annotations they process. When a match is found, the processor will be invoked. A processor may «claim» the annotations it processes, in which case no further attempt is made to find any processors for those annotations. Once all annotations have been claimed, the compiler does not look for additional processors.

If any processors generate any new source files, another round of annotation processing will occur: any newly generated source files will be scanned, and the annotations processed as before. Any processors invoked on previous rounds will also be invoked on all subsequent rounds. This continues until no new source files are generated.

After a round occurs where no new source files are generated, the annotation processors will be invoked one last time, to give them a chance to complete any work they may need to do. Finally, unless the -proc:only option is used, the compiler will compile the original and all the generated source files.

Implicitly loaded source files

To compile a set of source files, the compiler may need to implicitly load additional source files. (See ). Such files are currently not subject to annotation processing. By default, the compiler will give a warning if annotation processing has occurred and any implicitly loaded source files are compiled. See the option for ways to suppress the warning.