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"nb-javac" is a patched version of OpenJDK "javac", i.e., the Java compiler. This has long been part of NetBeans, providing a highly tuned Java compiler specifically for the Java editor i.e., parsing and lexing for features such as syntax coloring, code completion.
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/*
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 * DO NOT ALTER OR REMOVE COPYRIGHT NOTICES OR THIS FILE HEADER.
 *
 * This code is free software; you can redistribute it and/or modify it
 * under the terms of the GNU General Public License version 2 only, as
 * published by the Free Software Foundation.  Oracle designates this
 * particular file as subject to the "Classpath" exception as provided
 * by Oracle in the LICENSE file that accompanied this code.
 *
 * This code is distributed in the hope that it will be useful, but WITHOUT
 * ANY WARRANTY; without even the implied warranty of MERCHANTABILITY or
 * FITNESS FOR A PARTICULAR PURPOSE.  See the GNU General Public License
 * version 2 for more details (a copy is included in the LICENSE file that
 * accompanied this code).
 *
 * You should have received a copy of the GNU General Public License version
 * 2 along with this work; if not, write to the Free Software Foundation,
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package javax.tools;
/**
 * Provides methods for locating tool providers, for example,
 * providers of compilers.  This class complements the
 * functionality of {@link java.util.ServiceLoader}.
 *
 * @author Peter von der Ahé
 * @since 1.6
 */
public class ToolProvider {

    private static final String systemJavaCompilerModule = "jdk.compiler";
    private static final String systemJavaCompilerName   = "com.sun.tools.javac.api.JavacTool";

    private ToolProvider() {}
    /**
     * Returns the Java programming language compiler provided
     * with this platform.
     * The file manager returned by calling
     * {@link JavaCompiler#getStandardFileManager getStandardFileManager}
     * on this compiler supports paths provided by any
     * {@linkplain java.nio.file.FileSystem filesystem}.
     * @return the compiler provided with this platform or
     * {@code null} if no compiler is provided
     * @implNote This implementation returns the compiler provided
     * by the {@code jdk.compiler} module if that module is available,
     * and {@code null} otherwise.
     */
    public static JavaCompiler getSystemJavaCompiler() {
        return getSystemTool(JavaCompiler.class,
                systemJavaCompilerModule, systemJavaCompilerName);
    }

    private static final String systemDocumentationToolModule = "jdk.javadoc";
    private static final String systemDocumentationToolName = "jdk.javadoc.internal.api.JavadocTool";

    /**
     * Returns the Java programming language documentation tool provided
     * with this platform.
     * The file manager returned by calling
     * {@link DocumentationTool#getStandardFileManager getStandardFileManager}
     * on this tool supports paths provided by any
     * {@linkplain java.nio.file.FileSystem filesystem}.
     * @return the documentation tool provided with this platform or
     * {@code null} if no documentation tool is provided
     * @implNote This implementation returns the tool provided
     * by the {@code jdk.javadoc} module if that module is available,
     * and {@code null} otherwise.
     */
    public static DocumentationTool getSystemDocumentationTool() {
        return getSystemTool(DocumentationTool.class,
                systemDocumentationToolModule, systemDocumentationToolName);
    }

    /**
     * Returns a class loader that may be used to load system tools,
     * or {@code null} if no such special loader is provided.
     * @implSpec This implementation always returns {@code null}.
     * @deprecated This method is subject to removal in a future version of
     * Java SE.
     * Use the {@link java.util.spi.ToolProvider system tool provider} or
     * {@link java.util.ServiceLoader service loader} mechanisms to
     * locate system tools as well as user-installed tools.
     * @return a class loader, or {@code null}
     */
    @Deprecated(since="9")
    public static ClassLoader getSystemToolClassLoader() {
        return null;
    }

    /**
     * Get an instance of a system tool using the service loader.
     * @implNote         By default, this returns the implementation in the specified module.
     *                   For limited backward compatibility, if this code is run on an older version
     *                   of the Java platform that does not support modules, this method will
     *                   try and create an instance of the named class. Note that implies the
     *                   class must be available on the system class path.
     * @param         the interface of the tool
     * @param clazz      the interface of the tool
     * @param moduleName the name of the module containing the desired implementation
     * @param className  the class name of the desired implementation
     * @return the specified implementation of the tool
     */
    private static  T getSystemTool(Class clazz, String moduleName, String className) {
        try {
            return Class.forName(className, true, Thread.currentThread().getContextClassLoader()).
                asSubclass(clazz).getConstructor().newInstance();
        } catch (ReflectiveOperationException e) {
            throw new Error(e);
        }
    }
}