MaxCompute MapReduce及UDF程序在分布式环境中运行时，受到Java沙箱的限制（MapReduce作业的主程序，例如MapReduce Main则不受此限制）。
Java沙箱限制说明
- 不允许直接访问本地文件，只能通过MaxCompute MapReduce、Graph提供的接口间接访问。包括：
- Resources选项指定的资源，包括文件、Jar包和资源表等。
- 通过System.out和System.err输出日志信息。可以通过MaxCompute客户端的Log命令查看日志信息。
- 不允许直接访问分布式文件系统，只能通过MaxCompute MapReduce、Graph访问表的记录。
- 不允许JNI调用限制。
- 不允许创建Java线程，不允许启动子进程执行Linux命令。
- 不允许访问网络，包括获取本地IP地址等。
- Java反射限制：suppressAccessChecks权限被禁止，无法setAccessible某个私有的属性或方法，以达到读取私有属性或调用私有方法的目的。
访问本地文件限制
在代码中，直接使用下列方法访问本地文件会报access denied异常：
- java.io.File
public boolean delete() public void deleteOnExit() public boolean exists() public boolean canRead() public boolean isFile() public boolean isDirectory() public boolean isHidden() public long lastModified() public long length() public String[] list() public String[] list(FilenameFilter filter) public File[] listFiles() public File[] listFiles(FilenameFilter filter) public File[] listFiles(FileFilter filter) public boolean canWrite() public boolean createNewFile() public static File createTempFile(String prefix, String suffix) public static File createTempFile(String prefix, String suffix,File directory) public boolean mkdir() public boolean mkdirs() public boolean renameTo(File dest) public boolean setLastModified(long time) public boolean setReadOnly() - java.io.RandomAccessFile
RandomAccessFile(String name, String mode) RandomAccessFile(File file, String mode) - java.io.FileInputStream
FileInputStream(FileDescriptor fdObj) FileInputStream(String name) FileInputStream(File file) - java.io.FileOutputStream
FileOutputStream(FileDescriptor fdObj) FileOutputStream(File file) FileOutputStream(String name) FileOutputStream(String name, boolean append) - java.lang.Class
public ProtectionDomain getProtectionDomain() - java.lang.ClassLoader
ClassLoader() ClassLoader(ClassLoader parent) - java.lang.Runtime
public Process exec(String command) public Process exec(String command, String envp[]) public Process exec(String cmdarray[]) public Process exec(String cmdarray[], String envp[]) public void exit(int status) public static void runFinalizersOnExit(boolean value) public void addShutdownHook(Thread hook) public boolean removeShutdownHook(Thread hook) public void load(String lib) public void loadLibrary(String lib) - java.lang.System
public static void exit(int status) public static void runFinalizersOnExit(boolean value) public static void load(String filename) public static void loadLibrary( String libname) public static Properties getProperties() public static void setProperties(Properties props) public static String getProperty(String key) // 只允许部分key可以访问 public static String getProperty(String key, String def) // 只允许部分key可以访问 public static String setProperty(String key, String value) public static void setIn(InputStream in) public static void setOut(PrintStream out) public static void setErr(PrintStream err) public static synchronized void setSecurityManager(SecurityManager s)System.getProperty允许的key列表如下。java.version java.vendor java.vendor.url java.class.version os.name os.version os.arch file.separator path.separator line.separator java.specification.version java.specification.vendor java.specification.name java.vm.specification.version java.vm.specification.vendor java.vm.specification.name java.vm.version java.vm.vendor java.vm.name file.encoding user.timezone - java.lang.Thread
Thread() Thread(Runnable target) Thread(String name) Thread(Runnable target, String name) Thread(ThreadGroup group, ...) public final void checkAccess() public void interrupt() public final void suspend() public final void resume() public final void setPriority (int newPriority) public final void setName(String name) public final void setDaemon(boolean on) public final void stop() public final synchronized void stop(Throwable obj) public static int enumerate(Thread tarray[]) public void setContextClassLoader(ClassLoader cl) - java.lang.ThreadGroup
ThreadGroup(String name) ThreadGroup(ThreadGroup parent, String name) public final void checkAccess() public int enumerate(Thread list[]) public int enumerate(Thread list[], boolean recurse) public int enumerate(ThreadGroup list[]) public int enumerate(ThreadGroup list[], boolean recurse) public final ThreadGroup getParent() public final void setDaemon(boolean daemon) public final void setMaxPriority(int pri) public final void suspend() public final void resume() public final void destroy() public final void interrupt() public final void stop() - java.lang.reflect.AccessibleObject
public static void setAccessible(...) public void setAccessible(...) - java.net.InetAddress
public String getHostName() public static InetAddress[] getAllByName(String host) public static InetAddress getLocalHost() - java.net.DatagramSocket
public InetAddress getLocalAddress() - java.net.Socket
Socket(...) - java.net.ServerSocket
ServerSocket(...) public Socket accept() protected final void implAccept(Socket s) public static synchronized void setSocketFactory(...) public static synchronized void setSocketImplFactory(...) - java.net.DatagramSocket
DatagramSocket(...) public synchronized void receive(DatagramPacket p) - java.net.MulticastSocket
MulticastSocket(...) - java.net.URL
URL(...) public static synchronized void setURLStreamHandlerFactory(...) java.net.URLConnection public static synchronized void setContentHandlerFactory(...) public static void setFileNameMap(FileNameMap map) - java.net.HttpURLConnection
public static void setFollowRedirects(boolean set) java.net.URLClassLoader URLClassLoader(...) - java.security.AccessControlContext
public AccessControlContext(AccessControlContext acc, DomainCombiner combiner) public DomainCombiner getDomainCombiner()