Question

I am using Runtime.getRuntime().exec() command to start a batch file which in turn starts another process for windows platform.

javaw.exe(Process1)
 |___xyz.bat(Process2)
        |___javaw.exe(Process3)

Runtime.getRuntime().exec() returns a Process object which has a destroy method, but when I use destroy(), it kills only the xyz.bat and leaves the batch file’s sub-process dangling.

Is there a clean way in to destroy the process tree starting with the batch process as root?

For the record I cannot use any custom libraries to get rid of the batch file to by-pass the issue.

Answer 1

This is not possible using the standard Java API (see edit at end of the post for an update that changes this). You will need some native code of some variety. Using JNA, I’ve used code that looks like this:

public class Win32Process
{
    WinNT.HANDLE handle;
    int pid;

    Win32Process (int pid) throws IOException
    {
        handle = Kernel32.INSTANCE.OpenProcess ( 
                0x0400| /* PROCESS_QUERY_INFORMATION */
                0x0800| /* PROCESS_SUSPEND_RESUME */
                0x0001| /* PROCESS_TERMINATE */
                0x00100000 /* SYNCHRONIZE */,
                false,
                pid);
        if (handle == null) 
            throw new IOException ("OpenProcess failed: " + 
                    Kernel32Util.formatMessageFromLastErrorCode (Kernel32.INSTANCE.GetLastError ()));
        this.pid = pid;
    }

    @Override
    protected void finalize () throws Throwable
    {
        Kernel32.INSTANCE.CloseHandle (handle);
    }

    public void terminate ()
    {
        Kernel32.INSTANCE.TerminateProcess (handle, 0);
    }

    public List<Win32Process> getChildren () throws IOException
    {
        ArrayList<Win32Process> result = new ArrayList<Win32Process> ();
        WinNT.HANDLE hSnap = KernelExtra.INSTANCE.CreateToolhelp32Snapshot (KernelExtra.TH32CS_SNAPPROCESS, new DWORD(0));
        KernelExtra.PROCESSENTRY32.ByReference ent = new KernelExtra.PROCESSENTRY32.ByReference ();
        if (!KernelExtra.INSTANCE.Process32First (hSnap, ent)) return result;
        do {
            if (ent.th32ParentProcessID.intValue () == pid) result.add (new Win32Process (ent.th32ProcessID.intValue ()));
        } while (KernelExtra.INSTANCE.Process32Next (hSnap, ent));
        Kernel32.INSTANCE.CloseHandle (hSnap);
        return result;
    }

}

This code uses the following JNA declarations that are not included in the standard JNA library:

public interface KernelExtra extends StdCallLibrary {

    /**
     * Includes all heaps of the process specified in th32ProcessID in the snapshot. To enumerate the heaps, see
     * Heap32ListFirst.
     */
    WinDef.DWORD TH32CS_SNAPHEAPLIST = new WinDef.DWORD(0x00000001);

    /**
     * Includes all processes in the system in the snapshot. To enumerate the processes, see Process32First.
     */
    WinDef.DWORD TH32CS_SNAPPROCESS  = new WinDef.DWORD(0x00000002);

    /**
     * Includes all threads in the system in the snapshot. To enumerate the threads, see Thread32First.
     */
    WinDef.DWORD TH32CS_SNAPTHREAD   = new WinDef.DWORD(0x00000004);

    /**
     * Includes all modules of the process specified in th32ProcessID in the snapshot. To enumerate the modules, see
     * Module32First. If the function fails with ERROR_BAD_LENGTH, retry the function until it succeeds.
     */
    WinDef.DWORD TH32CS_SNAPMODULE   = new WinDef.DWORD(0x00000008);

    /**
     * Includes all 32-bit modules of the process specified in th32ProcessID in the snapshot when called from a 64-bit
     * process. This flag can be combined with TH32CS_SNAPMODULE or TH32CS_SNAPALL. If the function fails with
     * ERROR_BAD_LENGTH, retry the function until it succeeds.
     */
    WinDef.DWORD TH32CS_SNAPMODULE32 = new WinDef.DWORD(0x00000010);

    /**
     * Includes all processes and threads in the system, plus the heaps and modules of the process specified in th32ProcessID.
     */
    WinDef.DWORD TH32CS_SNAPALL      = new WinDef.DWORD((TH32CS_SNAPHEAPLIST.intValue() |
            TH32CS_SNAPPROCESS.intValue() | TH32CS_SNAPTHREAD.intValue() | TH32CS_SNAPMODULE.intValue()));

    /**
     * Indicates that the snapshot handle is to be inheritable.
     */
    WinDef.DWORD TH32CS_INHERIT      = new WinDef.DWORD(0x80000000);

    /**
     * Describes an entry from a list of the processes residing in the system address space when a snapshot was taken.
     */
    public static class PROCESSENTRY32 extends Structure {

        public static class ByReference extends PROCESSENTRY32 implements Structure.ByReference {
            public ByReference() {
            }

            public ByReference(Pointer memory) {
                super(memory);
            }
        }

        public PROCESSENTRY32() {
            dwSize = new WinDef.DWORD(size());
        }

        public PROCESSENTRY32(Pointer memory) {
            useMemory(memory);
            read();
        }

        /**
         * The size of the structure, in bytes. Before calling the Process32First function, set this member to
         * sizeof(PROCESSENTRY32). If you do not initialize dwSize, Process32First fails.
         */
        public WinDef.DWORD dwSize;

        /**
         * This member is no longer used and is always set to zero.
         */
        public WinDef.DWORD cntUsage;

        /**
         * The process identifier.
         */
        public WinDef.DWORD th32ProcessID;

        /**
         * This member is no longer used and is always set to zero.
         */
        public BaseTSD.ULONG_PTR th32DefaultHeapID;

        /**
         * This member is no longer used and is always set to zero.
         */
        public WinDef.DWORD th32ModuleID;

        /**
         * The number of execution threads started by the process.
         */
        public WinDef.DWORD cntThreads;

        /**
         * The identifier of the process that created this process (its parent process).
         */
        public WinDef.DWORD th32ParentProcessID;

        /**
         * The base priority of any threads created by this process.
         */
        public WinDef.LONG pcPriClassBase;

        /**
         * This member is no longer used, and is always set to zero.
         */
        public WinDef.DWORD dwFlags;

        /**
         * The name of the executable file for the process. To retrieve the full path to the executable file, call the
         * Module32First function and check the szExePath member of the MODULEENTRY32 structure that is returned.
         * However, if the calling process is a 32-bit process, you must call the QueryFullProcessImageName function to
         * retrieve the full path of the executable file for a 64-bit process.
         */
        public char[] szExeFile = new char[WinDef.MAX_PATH];
    }


    // the following methods are in kernel32.dll, but not declared there in the current version of Kernel32:

    /**
     * Takes a snapshot of the specified processes, as well as the heaps, modules, and threads used by these processes.
     *  
     * @param dwFlags
     *   The portions of the system to be included in the snapshot.
     * 
     * @param th32ProcessID
     *   The process identifier of the process to be included in the snapshot. This parameter can be zero to indicate
     *   the current process. This parameter is used when the TH32CS_SNAPHEAPLIST, TH32CS_SNAPMODULE,
     *   TH32CS_SNAPMODULE32, or TH32CS_SNAPALL value is specified. Otherwise, it is ignored and all processes are
     *   included in the snapshot.
     *
     *   If the specified process is the Idle process or one of the CSRSS processes, this function fails and the last
     *   error code is ERROR_ACCESS_DENIED because their access restrictions prevent user-level code from opening them.
     *
     *   If the specified process is a 64-bit process and the caller is a 32-bit process, this function fails and the
     *   last error code is ERROR_PARTIAL_COPY (299).
     *
     * @return
     *   If the function succeeds, it returns an open handle to the specified snapshot.
     *
     *   If the function fails, it returns INVALID_HANDLE_VALUE. To get extended error information, call GetLastError.
     *   Possible error codes include ERROR_BAD_LENGTH.
     */
    public WinNT.HANDLE CreateToolhelp32Snapshot(WinDef.DWORD dwFlags, WinDef.DWORD th32ProcessID);

    /**
     * Retrieves information about the first process encountered in a system snapshot.
     *
     * @param hSnapshot A handle to the snapshot returned from a previous call to the CreateToolhelp32Snapshot function.
     * @param lppe A pointer to a PROCESSENTRY32 structure. It contains process information such as the name of the
     *   executable file, the process identifier, and the process identifier of the parent process.
     * @return
     *   Returns TRUE if the first entry of the process list has been copied to the buffer or FALSE otherwise. The
     *   ERROR_NO_MORE_FILES error value is returned by the GetLastError function if no processes exist or the snapshot
     *   does not contain process information.
     */
    public boolean Process32First(WinNT.HANDLE hSnapshot, KernelExtra.PROCESSENTRY32.ByReference lppe);

    /**
     * Retrieves information about the next process recorded in a system snapshot.
     *
     * @param hSnapshot A handle to the snapshot returned from a previous call to the CreateToolhelp32Snapshot function.
     * @param lppe A pointer to a PROCESSENTRY32 structure.
     * @return
     *   Returns TRUE if the next entry of the process list has been copied to the buffer or FALSE otherwise. The
     *   ERROR_NO_MORE_FILES error value is returned by the GetLastError function if no processes exist or the snapshot
     *   does not contain process information.
     */
    public boolean Process32Next(WinNT.HANDLE hSnapshot, KernelExtra.PROCESSENTRY32.ByReference lppe);


}

You can then use the ‘getChildren()’ method to get a list of children, terminate the parent, and then recursively terminate the children.

I believe you can extra the PID of a java.lang.Process using reflection (I haven’t done this, however; I switched to creating the processes myself using the Win32 API so that I would have more control over it).

So putting it together, you’d need something like:

int pid = (some code to extract PID from the process you want to kill);
Win32Process process = new Win32Process(pid);
kill(process);

public void kill(Win32Process target) throws IOException
{
   List<Win32Process> children = target.getChildren ();
   target.terminateProcess ();
   for (Win32Process child : children) kill(child);
}

Edit

It turns out that this particular shortcoming of the Java API is being fixed in Java 9. See the preview of the Java 9 documentation here (if the correct page doesn’t load, you need to look at the java.lang.ProcessHandle interface). For the requirement of the question above, the code would now look something like this:

Process child = ...;
kill (child.toHandle());

public void kill (ProcessHandle handle)
{
    handle.descendants().forEach((child) -> kill(child));
    handle.destroy();
}

(Note that this isn’t tested – I haven’t switched to Java 9 yet, but am actively reading about it)