Subsystems: Improved exception handling for Java (DRAFT)

1
Subsystems Improved exception handling for Java
(DRAFT)

• Bart Jacobs, Frank Piessens

2
Outline

• Try-Catch Problem Solution
• Locking Problem Solution
• Cancellation Problem Solution
• Cleaning Up Problem Solution

3
Outline

• Try-Catch Problem Solution
• Locking Problem Solution
• Cancellation Problem Solution
• Cleaning Up Problem Solution

4
Problem StatementA Typical Try-Catch Pattern

• class Widget
• class WidgetManager
• int count
• Widget widgets new Widget10
• Widget allocWidget()
• Widget w new Widget()
• count
• if (count widgets.length 1)
• Widget ws new Widgetcount 2
• System.arraycopy(widgets, 0, ws, 0,
• widgets.length)
• widgets ws
• widgetscount 1 w
• return w
• void removeWidget(Widget w)

• class Program
• public static void main(String args)
• WidgetManager m new WidgetManager()
• while (true)
• String cmd getUserCommand()
• try
• m.allocWidget()
• m.removeWidget()
• catch (Throwable t)
• showErrorMessage(t)

This program is broken! Why?
5
Problem StatementAn OutOfMemoryError

• class Widget
• class WidgetManager
• int count
• Widget widgets new Widget10
• Widget allocWidget()
• Widget w new Widget()
• count
• if (count widgets.length 1)
• Widget ws new Widgetcount 2
• System.arraycopy(widgets, 0, ws, 0,
• widgets.length)
• widgets ws
• widgetscount 1 w
• return w
• void removeWidget(Widget w)

• class Program
• public static void main(String args)
• WidgetManager m new WidgetManager()
• while (true)
• String cmd getUserCommand()
• try
• m.allocWidget()
• m.removeWidget()
• catch (Throwable t)
• showErrorMessage(t)

m.count 11 m.widgets.length 10
6
Problem StatementAn OutOfMemoryError

• class Widget
• class WidgetManager
• int count
• Widget widgets new Widget10
• Widget allocWidget()
• Widget w new Widget()
• count
• if (count widgets.length 1)
• Widget ws new Widgetcount 2
• System.arraycopy(widgets, 0, ws, 0,
• widgets.length)
• widgets ws
• widgetscount 1 w
• return w
• void removeWidget(Widget w)

• class Program
• public static void main(String args)
• WidgetManager m new WidgetManager()
• while (true)
• String cmd getUserCommand()
• try
• m.allocWidget()
• m.removeWidget()
• catch (Throwable t)
• showErrorMessage(t)

3
4
1
2
m.count 11 m.widgets.length 10
7
Problem StatementAn OutOfMemoryError

• class Widget
• class WidgetManager
• int count
• Widget widgets new Widget10
• Widget allocWidget()
• Widget w new Widget()
• count
• if (count widgets.length 1)
• Widget ws new Widgetcount 2
• System.arraycopy(widgets, 0, ws, 0,
• widgets.length)
• widgets ws
• widgetscount 1 w
• return w
• void removeWidget(Widget w)

• class Program
• public static void main(String args)
• WidgetManager m new WidgetManager()
• while (true)
• String cmd getUserCommand()
• try
• m.allocWidget()
• m.removeWidget()
• catch (Throwable t)
• showErrorMessage(t)

3
4
1
2
5
m.count 12 m.widgets.length 10
8
Problem StatementA Typical Try-Catch Pattern

• class Widget
• class WidgetManager
• int count
• Widget widgets new Widget10
• Widget allocWidget()
• Widget w new Widget()
• count
• if (count widgets.length 1)
• Widget ws new Widgetcount 2
• System.arraycopy(widgets, 0, ws, 0,
• widgets.length)
• widgets ws
• widgetscount 1 w
• return w
• void removeWidget(Widget w)

• class Program
• public static void main(String args)
• WidgetManager m new WidgetManager()
• while (true)
• String cmd getUserCommand()
• try
• m.allocWidget()
• m.removeWidget()
• catch (Throwable t)
• showErrorMessage(t)

9
Proposed SolutionRe-entering the Owner Subsystem

• class WidgetManager
• Subsystem s Subsystem.getCurrent()
• int count
• Widget widgets new Widget10
• Widget allocWidget()
• reenter (s)
• Widget w new Widget()
• count
• if (count widgets.length 1)
• Widget ws new Widgetcount
  2
• System.arraycopy(widgets, 0, ws,
  0,
• widgets.length)
• widgets ws
• widgetscount 1 w
• return w
• void removeWidget(Widget w)

• class Program
• public static void main(String args)
• WidgetManager m new WidgetManager()
• while (true)
• String cmd getUserCommand()
• try
• m.allocWidget()
• m.removeWidget()
• catch (Throwable t)
• showErrorMessage(t)

child subsystem
root subsystem
10
Proposed SolutionRe-entering an Outer Subsystem

• class Program
• public static void main(String args)
• WidgetManager m new WidgetManager()
• while (true)
• String cmd getUserCommand()
• try
• m.allocWidget()
• m.removeWidget()
• catch (Throwable t)
• showErrorMessage(t)

• class WidgetManager
• Subsystem s Subsystem.getCurrent()
• int count
• Widget widgets new Widget10
• Widget allocWidget()
• reenter (s)
• Widget w new Widget()
• count
• if (count widgets.length 1)
• Widget ws new Widgetcount
  2
• System.arraycopy(widgets, 0, ws,
  0,
• widgets.length)
• widgets ws
• widgetscount 1 w
• return w
• void removeWidget(Widget w)

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Subsystems Basic Operation

• class Program
• public static void main(String args)
• Subsystem s Subsystem.getCurrent()
• try
• reenter (s)
• throw new RuntimeException()
• catch (Throwable t)
• System.out.println(t was
  caught.)

Not caught
12
Subsystems Basic Operation

• class Program
• public static void main(String args)
• try
• Subsystem s Subsystem.getCurrent()
• try
• reenter (s)
• throw new RuntimeException()
• catch (Throwable t)
• System.out.println(t caught
  by inner.)
• catch (Throwable t)
• System.out.println(t caught by
  outer.)

Caught by outer
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Subsystems Implementation

• class Subsystem
• static StackltSubsystemgt stack new
  StackltSubsystemgt()
• static stack.push(new Subsystem()
• Throwable exception
• Subsystem parent
• ListltSubsystemgt children new
  ArrayListltSubsystemgt()
• static Subsystem getCurrent() return
  stack.peek()
• static void enterNew()
• Subsystem s new Subsystem()
• s.parent getCurrent()
  getCurrent().children.add(s)
• stack.push(s)
• static void reenter(Subsystem s)
• s.checkNotFailed() stack.push(s)
• static void exit(Throwable e)
• if (e ! null) getCurrent().setFailed(e)
• stack.pop() getCurrent().checkNotFailed()
  
• void setFailed(Throwable e)

• Expansion try S catch (Throwable e) S
  
• Subsystem.enterNew()
• Throwable t null
• try
• S
• catch (Throwable e)
• t e
• Subsystem.finish(t)
• if (t ! null) Throwable e t S
• Expansion reenter (s) S
• Subsystem.reenter(s)
• Throwable t null
• try
• S
• catch (Throwable e)
• t e

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Outline

• Try-Catch Problem Solution
• Locking Problem Solution
• Cancellation Problem Solution
• Cleaning Up Problem Solution

15
Problem StatementA Typical Locking Pattern

• class Widget
• class WidgetManager
• int count
• Widget widgets new Widget10
• synchronized Widget allocWidget()
• Widget w new Widget()
• count
• if (count widgets.length 1)
• Widget ws new Widgetcount 2
• System.arraycopy(widgets, 0, ws, 0,
• widgets.length)
• widgets ws
• widgetscount 1 w
• return w
• synchronized void

• class Program
• public static void main(String args)
• final WidgetManager m new
  WidgetManager()
• while (true)
• String cmd getUserCommand()
• new Thread()
• public void run()
• try
• m.allocWidget()
• m.removeWidget()
• catch (Throwable t)
• showErrorMessage(t)
• .start()

This program is broken! Why?
16
Problem StatementAn OutOfMemoryError

• class Widget
• class WidgetManager
• int count
• Widget widgets new Widget10
• synchronized Widget allocWidget()
• Widget w new Widget()
• count
• if (count widgets.length 1)
• Widget ws new Widgetcount 2
• System.arraycopy(widgets, 0, ws, 0,
• widgets.length)
• widgets ws
• widgetscount 1 w
• return w
• synchronized void

• class Program
• public static void main(String args)
• final WidgetManager m new
  WidgetManager()
• while (true)
• String cmd getUserCommand()
• new Thread()
• public void run()
• try
• m.allocWidget()
• m.removeWidget()
• catch (Throwable t)
• showErrorMessage(t)
• .start()

In thread 1
m.count 11 m.widgets.length 10
17
Problem StatementAn OutOfMemoryError

• class Widget
• class WidgetManager
• int count
• Widget widgets new Widget10
• synchronized Widget allocWidget()
• Widget w new Widget()
• count
• if (count widgets.length 1)
• Widget ws new Widgetcount 2
• System.arraycopy(widgets, 0, ws, 0,
• widgets.length)
• widgets ws
• widgetscount 1 w
• return w
• synchronized void

• class Program
• public static void main(String args)
• final WidgetManager m new
  WidgetManager()
• while (true)
• String cmd getUserCommand()
• new Thread()
• public void run()
• try
• m.allocWidget()
• m.removeWidget()
• catch (Throwable t)
• showErrorMessage(t)
• .start()

In thread 1
In thread 2
m.count 12 m.widgets.length 10
18
Subsystems To The Rescue

• class WidgetManager
• Subsystem s Subsystem.getCurrent()
• int count
• Widget widgets new Widget10
• synchronized Widget allocWidget()
• reenter (s)
• Widget w new Widget()
• count
• if (count widgets.length 1)
• Widget ws new Widgetcount
  2
• System.arraycopy(widgets, 0, ws,
  0,
• widgets.length)
• widgets ws
• widgetscount 1 w
• return w

• class Program
• public static void main(String args)
• final WidgetManager m new
  WidgetManager()
• while (true)
• String cmd getUserCommand()
• new Thread()
• public void run()
• try
• m.allocWidget()
• m.removeWidget()
• catch (Throwable t)
• showErrorMessage(t)
• .start()

19
Subsystems To The Rescue

• class WidgetManager
• Subsystem s Subsystem.getCurrent()
• int count
• Widget widgets new Widget10
• synchronized Widget allocWidget()
• reenter (s)
• Widget w new Widget()
• count
• if (count widgets.length 1)
• Widget ws new Widgetcount
  2
• System.arraycopy(widgets, 0, ws,
  0,
• widgets.length)
• widgets ws
• widgetscount 1 w
• return w

• class Program
• public static void main(String args)
• final WidgetManager m new
  WidgetManager()
• while (true)
• String cmd getUserCommand()
• new Thread()
• public void run()
• try
• m.allocWidget()
• m.removeWidget()
• catch (Throwable t)
• showErrorMessage(t)
• .start()

In thread 1
20
Subsystems To The Rescue

• class WidgetManager
• Subsystem s Subsystem.getCurrent()
• int count
• Widget widgets new Widget10
• synchronized Widget allocWidget()
• reenter (s)
• Widget w new Widget()
• count
• if (count widgets.length 1)
• Widget ws new Widgetcount
  2
• System.arraycopy(widgets, 0, ws,
  0,
• widgets.length)
• widgets ws
• widgetscount 1 w
• return w

• class Program
• public static void main(String args)
• final WidgetManager m new
  WidgetManager()
• while (true)
• String cmd getUserCommand()
• new Thread()
• public void run()
• try
• m.allocWidget()
• m.removeWidget()
• catch (Throwable t)
• showErrorMessage(t)
• .start()

In thread 2
In thread 1
21
Subsystems To The Rescue
In main thread

• class WidgetManager
• Subsystem s Subsystem.getCurrent()
• int count
• Widget widgets new Widget10
• synchronized Widget allocWidget()
• reenter (s)
• Widget w new Widget()
• count
• if (count widgets.length 1)
• Widget ws new Widgetcount
  2
• System.arraycopy(widgets, 0, ws,
  0,
• widgets.length)
• widgets ws
• widgetscount 1 w
• return w

• class Program
• public static void main(String args)
• final WidgetManager m new
  WidgetManager()
• while (true)
• String cmd getUserCommand()
• new Thread()
• public void run()
• try
• m.allocWidget()
• m.removeWidget()
• catch (Throwable t)
• showErrorMessage(t)
• .start()

In thread 2
In thread 1
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Subsystems

• When an exception occurs in a subsystem s
• For safety New attempts to enter s (or a
  descendant) rethrow the exception, and
• For liveness Computations executing in s (or a
  descendant) in other threads are stopped
• I.e. In each thread that is executing in s (or a
  descendant), an exception is thrown (using
  Thread.stop()), which is caught when leaving s

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Outline

• Try-Catch Problem Solution
• Locking Problem Solution
• Cancellation Problem Solution
• Cleaning Up Problem Solution

24
Problem StatementA Typical Cancellation Pattern

• class Program
• public static void main(String args)
• final WidgetManager m new
  WidgetManager()
• ListltTaskgt tasks new ArrayListltTaskgt()
• while (true)
• String cmd getUserCommand()
• if (cmd.equals(cancelAll))
• for (Task t tasks) t.cancel()
  continue
• final Task task new Task()
  tasks.add(task)
• new Thread()
• public void run()
• try
• task.setThread(Thread.curr
  entThread())
• m.allocWidget()
• m.removeWidget()
• catch (Throwable t)

• class Task
• boolean cancel
• Thread thread
• synchronized void cancel()
• cancel true if (thread ! null)
  thread.stop()
• synchronized void setThread(Thread t)
• thread t if (cancel) throw new
  ThreadDeath()
• class Widget
• class WidgetManager
• int count
• Widget widgets new Widget10
• synchronized Widget allocWidget()
• Widget w new Widget()
• count
• if (count widgets.length 1)
• Widget ws new Widgetcount 2
• System.arraycopy(widgets, 0, ws, 0,

This program is broken! Why?
25
Problem StatementA ThreadDeath

• class Program
• public static void main(String args)
• final WidgetManager m new
  WidgetManager()
• ListltTaskgt tasks new ArrayListltTaskgt()
• while (true)
• String cmd getUserCommand()
• if (cmd.equals(cancelAll))
• for (Task t tasks) t.cancel()
  continue
• final Task task new Task()
  tasks.add(task)
• new Thread()
• public void run()
• try
• task.setThread(Thread.curr
  entThread())
• m.allocWidget()
• m.removeWidget()
• catch (Throwable t)

• class Task
• boolean cancel
• Thread thread
• synchronized void cancel()
• cancel true if (thread ! null)
  thread.stop()
• synchronized void setThread(Thread t)
• thread t if (cancel) throw new
  ThreadDeath()
• class Widget
• class WidgetManager
• int count
• Widget widgets new Widget10
• synchronized Widget allocWidget()
• Widget w new Widget()
• count
• if (count widgets.length 1)
• Widget ws new Widgetcount 2
• System.arraycopy(widgets, 0, ws, 0,

ThreadDeath in thread 1
m.count 11 m.widgets.length 10
26
Problem StatementA ThreadDeath

• class Program
• public static void main(String args)
• final WidgetManager m new
  WidgetManager()
• ListltTaskgt tasks new ArrayListltTaskgt()
• while (true)
• String cmd getUserCommand()
• if (cmd.equals(cancelAll))
• for (Task t tasks) t.cancel()
  continue
• final Task task new Task()
  tasks.add(task)
• new Thread()
• public void run()
• try
• task.setThread(Thread.curr
  entThread())
• m.allocWidget()
• m.removeWidget()
• catch (Throwable t)

• class Task
• boolean cancel
• Thread thread
• synchronized void cancel()
• cancel true if (thread ! null)
  thread.stop()
• synchronized void setThread(Thread t)
• thread t if (cancel) throw new
  ThreadDeath()
• class Widget
• class WidgetManager
• int count
• Widget widgets new Widget10
• synchronized Widget allocWidget()
• Widget w new Widget()
• count
• if (count widgets.length 1)
• Widget ws new Widgetcount 2
• System.arraycopy(widgets, 0, ws, 0,

ThreadDeath in thread 1
In thread 2
m.count 12 m.widgets.length 10
27
Proposed SolutionSubsystem Cancellation

• class Program
• public static void main(String args)
• final WidgetManager m new
  WidgetManager()
• ListltTaskgt tasks new ArrayListltTaskgt()
• while (true)
• String cmd getUserCommand()
• if (cmd.equals(cancelAll))
• for (Task t tasks) t.cancel()
  continue
• final Task task new Task()
  tasks.add(task)
• new Thread()
• public void run()
• try
• task.setSubsystem(Subsyste
  m.getCurrent())
• m.allocWidget()
• m.removeWidget()
• catch (Throwable t)

• class Task
• boolean cancel
• Subsystem subsystem
• synchronized void cancel()
• cancel true if (subsystem ! null)
  subsystem.cancel()
• synchronized void setSubsystem(Subsystem s)
• subsystem s if (cancel) throw new
  ThreadDeath()
• class Widget
• class WidgetManager
• Subsystem s Subsystem.getCurrent()
• int count
• Widget widgets new Widget10
• synchronized Widget allocWidget()
• reenter (s)
• Widget w new Widget()
• count
• if (count widgets.length 1)

28
Proposed SolutionSubsystem Cancellation

• class Program
• public static void main(String args)
• final WidgetManager m new
  WidgetManager()
• ListltTaskgt tasks new ArrayListltTaskgt()
• while (true)
• String cmd getUserCommand()
• if (cmd.equals(cancelAll))
• for (Task t tasks) t.cancel()
  continue
• final Task task new Task()
  tasks.add(task)
• new Thread()
• public void run()
• try
• task.setSubsystem(Subsyste
  m.getCurrent())
• m.allocWidget()
• m.removeWidget()
• catch (Throwable t)

• class Task
• boolean cancel
• Subsystem subsystem
• synchronized void cancel()
• cancel true if (subsystem ! null)
  subsystem.cancel()
• synchronized void setSubsystem(Subsystem s)
• subsystem s if (cancel) throw new
  ThreadDeath()
• class Widget
• class WidgetManager
• Subsystem s Subsystem.getCurrent()
• int count
• Widget widgets new Widget10
• synchronized Widget allocWidget()
• reenter (s)
• Widget w new Widget()
• count
• if (count widgets.length 1)

(Child subsystem is cancelled)
ThreadDeath on entry to child
29
Outline

• Try-Catch Problem Solution
• Locking Problem Solution
• Cancellation Problem Solution
• Cleaning Up Problem Solution

30
Problem StatementA Typical Cleanup Pattern

• class Widget implements Closeable
• WidgetManager m
• Widget(WidgetManager m) this.m m
• public void close() m.removeWidget(this)
• class WidgetManager
• int count
• Widget widgets new Widget10
• Widget allocWidget()
• Widget w new Widget()
• count
• if (count widgets.length 1)
• Widget ws new Widgetcount 2
• System.arraycopy(widgets, 0, ws, 0,
• widgets.length)
• widgets ws

• class Program
• static Widget allocGreenWidget(WidgetManager
  m)
• Widget w m.allocWidget()
• w.setColor(Color.green)
• return w
• public static void main(String args)
• WidgetManager m new WidgetManager()
• while (true)
• String cmd getUserCommand()
• try
• Widget w allocGreenWidget(m)
• try
• finally w.close()
• catch (Throwable t)
• showErrorMessage(t)

This program is broken! Why?
31
Problem StatementA StackOverflowError

• class Widget implements Closeable
• WidgetManager m
• Widget(WidgetManager m) this.m m
• public void close() m.removeWidget(this)
• class WidgetManager
• int count
• Widget widgets new Widget10
• Widget allocWidget()
• Widget w new Widget()
• count
• if (count widgets.length 1)
• Widget ws new Widgetcount 2
• System.arraycopy(widgets, 0, ws, 0,
• widgets.length)
• widgets ws

• class Program
• static Widget allocGreenWidget(WidgetManager
  m)
• Widget w m.allocWidget()
• w.setColor(Color.green)
• return w
• public static void main(String args)
• WidgetManager m new WidgetManager()
• while (true)
• String cmd getUserCommand()
• try
• Widget w allocGreenWidget(m)
• try
• finally w.close()
• catch (Throwable t)
• showErrorMessage(t)

32
Proposed SolutionSubsystem Cleanup Routines

• class Widget implements Closeable
• Subsystem client Subsystem.getCaller()
• WidgetManager m
• Widget(WidgetManager m)
• client.registerCleanup(this) this.m m
• public void close() m.removeWidget(this)
• class WidgetManager
• Subsystem s Subsystem.getCurrent()
• int count
• Widget widgets new Widget10
• Widget allocWidget()
• reenter (s)
• Widget w new Widget()
• count
• if (count widgets.length 1)
• Widget ws new Widgetcount
  2
• System.arraycopy(widgets, 0, ws,
  0,

• class Program
• static Widget allocGreenWidget(WidgetManager
  m)
• Widget w m.allocWidget()
• w.setColor(Color.green)
• return w
• public static void main(String args)
• WidgetManager m new WidgetManager()
• while (true)
• String cmd getUserCommand()
• try
• Widget w allocGreenWidget(m)
• try
• finally w.close()
• catch (Throwable t)
• showErrorMessage(t)

33
Related Work(Under Construction)

• Re-entering Similar to performing a remote
  procedure call in systems where each subsystem is
  a separate process/thread
• Cancellation
• Rudys, Wallach. Termination in language-based
  systems. ACM TISS 5(2), 2002.
• Wick, Flatt. Memory accounting without
  partitions. ISMM 2004.
• Flatt, Findler. Kill-safe synchronization
  abstractions. PLDI 2004.
• Flatt, Findler, Krishnamurthi, Felleisen.
  Programming languages as operating systems. ICFP
  1999.
• Subsystem cleanup stacks
• Similar to compensation stacks in Weimer. Finding
  and preventing run-time error handling mistakes.
  OOPSLA 2004.

34
Conclusion

• Its hard in Java to catch unchecked exceptions
  safely
• Many (most?) existing try-catch blocks for
  unchecked exceptions are probably unsafe
• Many synchronized blocks are probably unsafe
• There is no easy and safe way to cancel a
  computation
• Subsystems make it easy to fix these problems for
  the example programs
• Future work Assess the severity of the problem
  and the effectiveness of subsystems in large
  programs