JDK14/Java14源码在线阅读

/*
 * Copyright (c) 2016, 2018, Oracle and/or its affiliates. All rights reserved.
 * 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,
 * Inc., 51 Franklin St, Fifth Floor, Boston, MA 02110-1301 USA.
 *
 * Please contact Oracle, 500 Oracle Parkway, Redwood Shores, CA 94065 USA
 * or visit www.oracle.com if you need additional information or have any
 * questions.
 */
package jdk.jfr.internal;

import java.io.IOException;

/**
 * Checks if the running VM supports Flight Recorder.
 *
 * Purpose of this helper class is to detect early and cleanly if the VM has
 * support for Flight Recorder, i.e. not throw {@link UnsatisfiedLinkError} in
 * unexpected places.
 * <p>
 * This is needed so a disabled-jfr.jar can be built for non Oracle JDKs.
 */
public final class JVMSupport {

    private static final String UNSUPPORTED_VM_MESSAGE = "Flight Recorder is not supported on this VM";
    private static final boolean notAvailable = !checkAvailability();

    private static boolean checkAvailability() {
        // set jfr.unsupported.vm to true to test API on an unsupported VM
        try {
            if (SecuritySupport.getBooleanProperty("jfr.unsupported.vm")) {
                return false;
            }
        } catch (NoClassDefFoundError cnfe) {
            // May happen on JDK 8, where jdk.internal.misc.Unsafe can't be found
            return false;
        }
        try {
            // Will typically throw UnsatisfiedLinkError if
            // there is no native implementation
            JVM.getJVM().isAvailable();
            return true;
        } catch (Throwable t) {
            return false;
        }
    }

    public static void ensureWithInternalError() {
        if (notAvailable) {
            throw new InternalError(UNSUPPORTED_VM_MESSAGE);
        }
    }

    public static void ensureWithIOException() throws IOException {
        if (notAvailable) {
            throw new IOException(UNSUPPORTED_VM_MESSAGE);
        }
    }


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