android-platform-libcore/luni/src/main/java/org/apache/harmony/luni/platform/Platform.java

/* Licensed to the Apache Software Foundation (ASF) under one or more
 * contributor license agreements.  See the NOTICE file distributed with
 * this work for additional information regarding copyright ownership.
 * The ASF licenses this file to You under the Apache License, Version 2.0
 * (the "License"); you may not use this file except in compliance with
 * the License.  You may obtain a copy of the License at
 *
 *     http://www.apache.org/licenses/LICENSE-2.0
 *
 * Unless required by applicable law or agreed to in writing, software
 * distributed under the License is distributed on an "AS IS" BASIS,
 * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
 * See the License for the specific language governing permissions and
 * limitations under the License.
 */

package org.apache.harmony.luni.platform;

import dalvik.system.BlockGuard;
import dalvik.system.VMStack;

/**
 * The Platform class gives access to the low-level underlying capabilities of
 * the operating system.
 *
 * The platform is structured into operations on the process heap memory,
 * network subsystem, and file system through different OS components.
 *
 * OS components are 'dangerous' in that they pass through the calls and
 * arguments to the OS with very little checking, and as such may cause fatal
 * exceptions in the runtime. Access to the OS components is restricted to
 * trusted code running on the system classpath.
 *
 * @see IFileSystem
 * @see INetworkSystem
 */
public class Platform {
    // Note: for now, we're always wrapping the filesystem with
    // BlockGuard.  In the future we intend to measure this and, with
    // the arriving of upcoming method call Dalvik improvements,
    // remove a lot of the static caching in RandomAccessFile, etc.,
    // at which point we can make getFileSystem() return an unwrapped
    // filesystem in some cases so RandomAccessFiles created on
    // BlockGuard-policy-free threads have no extra overhead.  But for
    // now they do: ThreadLocal lookups will be done on most VFS
    // operations, which should be relatively less than the speed of
    // the flash.
    // TODO: measure & fix if needed.
    private static final IFileSystem FILE_SYSTEM =
            new BlockGuard.WrappedFileSystem(OSFileSystem.getOSFileSystem());

    private static final INetworkSystem NETWORK_SYSTEM =
            new BlockGuard.WrappedNetworkSystem(OSNetworkSystem.getOSNetworkSystem());

    /**
     * Checks to ensure that whoever is asking for the OS component is running
     * on the system classpath.
     */
    private static final void accessCheck() {
        if (VMStack.getCallingClassLoader() != null) {
            throw new SecurityException();
        }
    }

    public static IFileSystem getFileSystem() {
        accessCheck();
        return FILE_SYSTEM;
    }

    public static INetworkSystem getNetworkSystem() {
        accessCheck();
        // TODO: use BlockGuard here too, like in getFileSystem() above.
        return NETWORK_SYSTEM;
    }
}