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The Checker Framework enhances Java's type system to make it more powerful and useful. This lets software developers detect and prevent errors in their Java programs. The Checker Framework includes compiler plug-ins ("checkers") that find bugs or verify their absence. It also permits you to write your own compiler plug-ins.
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package org.checkerframework.checker.nullness;

import com.sun.source.tree.MethodInvocationTree;

import org.checkerframework.framework.type.AnnotatedTypeMirror;
import org.checkerframework.framework.type.AnnotatedTypeMirror.AnnotatedExecutableType;
import org.checkerframework.javacutil.TreeUtils;

import java.util.Arrays;
import java.util.Collection;
import java.util.HashSet;

import javax.annotation.processing.ProcessingEnvironment;
import javax.lang.model.element.ExecutableElement;

/**
 * Utility class for handling {@link java.lang.System#getProperty(String)} and related invocations.
 *
 * The result of the method call is assumed to be non-null if the argument is a literal key that
 * is guaranteed to be present in the system properties (according to the documentation of {@link
 * java.lang.System#getProperty(String)}), as in {@code System.getProperties("line.separator")}.
 */
public class SystemGetPropertyHandler {

    /**
     * If true, client code may clear system properties, and this class (SystemGetPropertyHandler)
     * has no effect.
     */
    private final boolean permitClearProperty;

    /** The processing environment. */
    private final ProcessingEnvironment env;

    /** The factory for constructing and looking up types. */
    private final NullnessNoInitAnnotatedTypeFactory factory;

    /** The System.getProperty(String) method. */
    private final ExecutableElement systemGetProperty;

    /** The System.setProperty(String) method. */
    private final ExecutableElement systemSetProperty;

    /**
     * System properties that are defined at startup on every JVM.
     *
     * This list is from the Javadoc of System.getProperties, for Java 17.
     */
    public static final Collection predefinedSystemProperties =
            new HashSet<>(
                    Arrays.asList(
                            "java.version",
                            "java.version.date",
                            "java.vendor",
                            "java.vendor.url",
                            "java.vendor.version",
                            "java.home",
                            "java.vm.specification.version",
                            "java.vm.specification.vendor",
                            "java.vm.specification.name",
                            "java.vm.version",
                            "java.vm.vendor",
                            "java.vm.name",
                            "java.specification.version",
                            "java.specification.maintenance.version",
                            "java.specification.vendor",
                            "java.specification.name",
                            "java.class.version",
                            "java.class.path",
                            "java.library.path",
                            "java.io.tmpdir",
                            "java.compiler",
                            "os.name",
                            "os.arch",
                            "os.version",
                            "file.separator",
                            "path.separator",
                            "line.separator",
                            "user.name",
                            "user.home",
                            "user.dir",
                            "native.encoding",
                            "stdout.encoding",
                            "stderr.encoding",
                            "jdk.module.path",
                            "jdk.module.upgrade.path",
                            "jdk.module.main",
                            "jdk.module.main.class",
                            "file.encoding"));

    /**
     * Creates a SystemGetPropertyHandler.
     *
     * @param env the processing environment
     * @param factory the factory for constructing and looking up types
     * @param permitClearProperty if true, client code may clear system properties, and this object
     *     does nothing
     */
    public SystemGetPropertyHandler(
            ProcessingEnvironment env,
            NullnessNoInitAnnotatedTypeFactory factory,
            boolean permitClearProperty) {
        this.env = env;
        this.factory = factory;
        this.permitClearProperty = permitClearProperty;

        systemGetProperty = TreeUtils.getMethod("java.lang.System", "getProperty", 1, env);
        systemSetProperty = TreeUtils.getMethod("java.lang.System", "setProperty", 2, env);
    }

    /**
     * Apply rules regarding System.getProperty and related methods.
     *
     * @param tree a method invocation
     * @param method the method being invoked
     */
    public void handle(MethodInvocationTree tree, AnnotatedExecutableType method) {
        if (permitClearProperty) {
            return;
        }
        if (TreeUtils.isMethodInvocation(tree, systemGetProperty, env)
                || TreeUtils.isMethodInvocation(tree, systemSetProperty, env)) {
            String literal = NullnessNoInitVisitor.literalFirstArgument(tree);
            if (literal != null && predefinedSystemProperties.contains(literal)) {
                AnnotatedTypeMirror type = method.getReturnType();
                type.replaceAnnotation(factory.NONNULL);
            }
        }
    }
}