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33<h1><a name="My_Project_"></a>Dalvik VM<br>Debug Monitor</h1>
34
35<!-- Status is one of: Draft, Current, Needs Update, Obsolete -->
36<p style="text-align:center"><strong>Status:</strong><em>Draft</em> &nbsp;
37<small>(as of March 6, 2007)</small></p>
38<address>
39[authors]
40<address>
41
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54</address>
55
56<p><br>
57<HR>
58
59<h2>Introduction</h2>
60
61<p>It's extremely useful to be able to monitor the live state of the
62VM.  For Android, we need to monitor multiple VMs running on a device
63connected through USB or a wireless network connection.  This document
64describes a debug monitor server that interacts with multiple VMs, and
65an API that VMs and applications can use to provide information
66to the monitor.
67
68<p>Some things we can monitor with the Dalvik Debug Monitor ("DDM"):
69<ul>
70    <li> Thread states.  Track thread creation/exit, busy/idle status.
71    <li> Overall heap status, useful for a heap bitmap display or
72    fragmentation analysis.
73</ul>
74
75<p>It is possible for something other than a VM to act as a DDM client, but
76that is a secondary goal.  Examples include "logcat" log extraction
77and system monitors for virtual memory usage and load average.
78
79<p>It's also possible for the DDM server to be run on the device, with
80the information presented through the device UI.  However, the initial goal
81is to provide a display tool that takes advantage of desktop tools and
82screen real estate.
83
84<p>This work is necessary because we are unable to use standard JVMTI-based
85tools with Dalvik.  JVMTI relies on bytecode insertion, which is not
86currently possible because Dalvik doesn't support Java bytecode.
87
88<p>The DDM server is written in the Java programming language
89for portability.  It uses a desktop
90UI toolkit (SWT) for its interface.
91
92
93<h2>Protocol</h2>
94
95<p>To take advantage of existing infrastructure we are piggy-backing the
96DDM protocol on top of JDWP (the Java Debug Wire Protocol, normally spoken
97between a VM and a debugger).  To a
98non-DDM client, the DDM server just looks like a debugger.
99
100<p>The JDWP protocol is very close to what we want to use.  In particular:
101<ul>
102    <li>It explicitly allows for vendor-defined packets, so there is no
103    need to "bend" the JDWP spec.
104    <li>Events may be posted from the VM at arbitrary points.  Such
105    events do not elicit a response from the debugger, meaning the client
106    can post data and immediately resume work without worrying about the
107    eventual response.
108    <li>The basic protocol is stateless and asynchronous.  Request packets
109    from the debugger side include a serial number, which the VM includes
110    in the response packet.  This allows multiple simultaneous
111    conversations, which means the DDM traffic can be interleaved with
112    debugger traffic.
113</ul>
114
115<p>There are a few issues with using JDWP for our purposes:
116<ul>
117    <li>The VM only expects one connection from a debugger, so you couldn't
118    attach the monitor and a debugger at the same time.  This will be
119    worked around by connecting the debugger to the monitor and passing the
120    traffic through.  (We're already doing the pass-through with "jdwpspy";
121    requires some management of our request IDs though.)  This should
122    be more convenient than the current "guess the port
123    number" system when we're attached to a device.
124    <li>The VM behaves differently when a debugger is attached.  It will
125    run more slowly, and any objects passed to the monitor or debugger are
126    immune to GC.  We can work around this by not enabling the slow path
127    until non-DDM traffic is observed.  We also want to have a "debugger
128    has connected/disconnected" message that allows the VM to release
129    debugger-related resources without dropping the net connection.
130    <li>Non-DDM VMs should not freak out when DDM connects.  There are
131    no guarantees here for 3rd-party VMs (e.g. a certain mainstream VM,
132    which crashes instantly), but our older JamVM can be
133    configured to reject the "hello" packet.
134</ul>
135
136
137<h3>Connection Establishment</h3>
138
139<p>There are two basic approaches: have the server contact the VMs, and
140have the VMs contact the server.  The former is less "precise" than the
141latter, because you have to scan for the clients, but it has some
142advantages.
143
144<p>There are three interesting scenarios:
145<ol>
146    <li>The DDM server is started, then the USB-attached device is booted
147    or the simulator is launched.
148    <li>The device or simulator is already running when the DDM server
149    is started.
150    <li>The DDM server is running when an already-started device is
151    attached to USB.
152</ol>
153<p>If we have the VMs connect to the DDM server on startup, we only handle
154case #1.  If the DDM server scans for VMs when it starts, we only handle
155case #2.  Neither handles case #3, which is probably the most important
156of the bunch as the device matures.
157<p>The plan is to have a drop-down menu with two entries,
158"scan workstation" and "scan device".
159The former causes the DDM server to search for VMs on "localhost", the
160latter causes it to search for VMs on the other side of an ADB connection.
161The DDM server will scan for VMs every few seconds, either checking a
162range of known VM ports (e.g. 8000-8040) or interacting with some sort
163of process database on the device.  Changing modes causes all existing
164connections to be dropped.
165<p>When the DDM server first starts, it will try to execute "adb usb"
166to ensure that the ADB server is running.  (Note it will be necessary
167to launch the DDM server from a shell with "adb" in the path.)  If this
168fails, talking to the device will still be possible so long as the ADB
169daemon is already running.
170
171<h4>Connecting a Debugger</h4>
172
173<p>With the DDM server sitting on the JDWP port of all VMs, it will be
174necessary to connect the debugger through the DDM server.  Each VM being
175debugged will have a separate port being listened to by the DDM server,
176allowing you to connect a debugger to one or more VMs simultaneously.
177
178<p>In the common case, however, the developer will only want to debug
179a single VM.  One port (say 8700) will be listened to by the DDM server,
180and anything connecting to it will be connected to the "current VM"
181(selected in the UI).  This should allow developers to focus on a
182single application, which may otherwise shift around in the ordering, without
183having to adjust their IDE settings to a different port every time they
184restart the device.
185
186
187<h3>Packet Format</h3>
188
189<p>Information is sent in chunks.  Each chunk starts with:
190<pre>
191u4   type
192u4   length
193</pre>
194and contains a variable amount of type-specific data.
195Unrecognized types cause an empty response from the client and
196are quietly ignored by the server.  [Should probably return an error;
197need an "error" chunk type and a handler on the server side.]
198
199<p>The same chunk type may have different meanings when sent in different
200directions.  For example, the same type may be used for both a query and
201a response to the query.  For sanity the type must always be used in
202related transactions.
203
204<p>This is somewhat redundant with the JDWP framing, which includes a
2054-byte length and a two-byte type code ("command set" and "command"; a
206range of command set values is designated for "vendor-defined commands
207and extensions").  Using the chunk format allows us to remain independent
208of the underlying transport, avoids intrusive integration
209with JDWP client code, and provides a way to send multiple chunks in a
210single transmission unit.  [I'm taking the multi-chunk packets into
211account in the design, but do not plan to implement them unless the need
212arises.]
213
214<p>Because we may be sending data over a slow USB link, the chunks may be
215compressed.  Compressed chunks are written as a chunk type that
216indicates the compression, followed by the compressed length, followed
217by the original chunk type and the uncompressed length.  For zlib's deflate
218algorithm, the chunk type is "ZLIB".
219
220<p>Following the JDWP model, packets sent from the server to the client
221are always acknowledged, but packets sent from client to server never are.
222The JDWP error code field is always set to "no error"; failure responses
223from specific requests must be encoded into the DDM messages.
224
225<p>In what follows "u4" is an unsigned 32-bit value and "u1" is an
226unsigned 8-bit value.  Values are written in big-endian order to match
227JDWP.
228
229
230<h3>Initial Handshake</h3>
231
232<p>After the JDWP handshake, the server sends a HELO chunk to the client.
233If the client's JDWP layer rejects it, the server assumes that the client
234is not a DDM-aware VM, and does not send it any further DDM queries.
235<p>On the client side, upon seeing a HELO it can know that a DDM server
236is attached and prepare accordingly.  The VM should not assume that a
237debugger is attached until a non-DDM packet arrives.
238
239<h4>Chunk HELO (server --&gt; client)</h4>
240<p>Basic "hello" message.
241<pre>
242u4   DDM server protocol version
243</pre>
244
245
246<h4>Chunk HELO (client --&gt; server, reply only)</h4>
247Information about the client.  Must be sent in response to the HELO message.
248<pre>
249u4   DDM client protocol version
250u4   pid
251u4   VM ident string len (in 16-bit units)
252u4   application name len (in 16-bit units)
253var  VM ident string (UTF-16)
254var  application name (UTF-16)
255</pre>
256
257<p>If the client does not wish to speak to the DDM server, it should respond
258with a JDWP error packet.  This is the same behavior you'd get from a VM
259that doesn't support DDM.
260
261
262<h3>Debugger Management</h3>
263<p>VMs usually prepare for debugging when a JDWP connection is established,
264and release debugger-related resources when the connection drops.  We want
265to open the JDWP connection early and hold it open after the debugger
266disconnects.
267<p>The VM can tell when a debugger attaches, because it will start seeing
268non-DDM JDWP traffic, but it can't identify the disconnect.  For this reason,
269we need to send a packet to the client when the debugger disconnects.
270<p>If the DDM server is talking to a non-DDM-aware client, it will be
271necessary to drop and re-establish the connection when the debugger goes away.
272(This also works with DDM-aware clients; this packet is an optimization.)
273
274<h4>Chunk DBGD (server --&gt; client)</h4>
275<p>Debugger has disconnected.  The client responds with a DBGD to acknowledge
276receipt.  No data in request, no response required.
277
278
279<h3>VM Info</h3>
280<p>Update the server's info about the client.
281
282<h4>Chunk APNM (client --&gt; server)</h4>
283
284<p>If a VM's application name changes -- possible in our environment because
285of the "pre-initialized" app processes -- it must send up one of these.
286<pre>
287u4   application name len (in 16-bit chars)
288var  application name (UTF-16)
289</pre>
290
291<h4>Chunk WAIT (client --&gt; server)</h4>
292
293<p>This tells DDMS that one or more threads are waiting on an external
294event.  The simplest use is to tell DDMS that the VM is waiting for a
295debugger to attach.
296<pre>
297u1   reason  (0 = wait for debugger)
298</pre>
299If DDMS is attached, the client VM sends this up when waitForDebugger()
300is called.  If waitForDebugger() is called before DDMS attaches, the WAIT
301chunk will be sent up at about the same time as the HELO response.
302
303
304<h3>Thread Status</h3>
305
306<p>The client can send updates when their status changes, or periodically
307send thread state info, e.g. 2x per
308second to allow a "blinkenlights" display of thread activity.
309
310<h4>Chunk THEN (server --&gt; client)</h4>
311
312<p>Enable thread creation/death notification.
313<pre>
314u1   boolean (true=enable, false=disable)
315</pre>
316<p>The response is empty.  The client generates THCR packets for all
317known threads.  (Note the THCR packets may arrive before the THEN
318response.)
319
320<h4>Chunk THCR (client --&gt; server)</h4>
321<p>Thread Creation notification.
322<pre>
323u4   VM-local thread ID (usually a small int)
324u4   thread name len (in 16-bit chars)
325var  thread name (UTF-16)
326</pre>
327
328<h4>Chunk THDE (client --&gt; server)</h4>
329<p>Thread Death notification.
330<pre>
331u4   VM-local thread ID
332</pre>
333
334<h4>Chunk THST (server --&gt; client)</h4>
335
336<p>Enable periodic thread activity updates.
337Threads in THCR messages are assumed to be in the "initializing" state.  A
338THST message should follow closely on the heels of THCR.
339<pre>
340u4   interval, in msec
341</pre>
342<p>An interval of 0 disables the updates.  This is done periodically,
343rather than every time the thread state changes, to reduce the amount
344of data that must be sent for an actively running VM.
345
346<h4>Chunk THST (client --&gt; server)</h4>
347<p>Thread Status, describing the state of one or more threads.  This is
348most useful when creation/death notifications are enabled first.  The
349overall layout is:
350<pre>
351u4   count
352var  thread data
353</pre>
354Then, for every thread:
355<pre>
356u4   VM-local thread ID
357u1   thread state
358u1   suspended
359</pre>
360<p>"thread state" must be one of:
361<ul>    <!-- don't use ol, we may need (-1) or sparse -->
362    <li> 1 - running (now executing or ready to do so)
363    <li> 2 - sleeping (in Thread.sleep())
364    <li> 3 - monitor (blocked on a monitor lock)
365    <li> 4 - waiting (in Object.wait())
366    <li> 5 - initializing
367    <li> 6 - starting
368    <li> 7 - native (executing native code)
369    <li> 8 - vmwait (waiting on a VM resource)
370</ul>
371<p>"suspended" will be 0 if the thread is running, 1 if not.
372<p>[Any reason not to make "suspended" be the high bit of "thread state"?
373Do we need to differentiate suspend-by-GC from suspend-by-debugger?]
374<p>[We might be able to send the currently-executing method.  This is a
375little risky in a running VM, and increases the size of the messages
376considerably, but might be handy.]
377
378
379<h3>Heap Status</h3>
380
381<p>The client sends what amounts to a color-coded bitmap to the server,
382indicating which stretches of memory are free and which are in use.  For
383compactness the bitmap is run-length encoded, and based on multi-byte
384"allocation units" rather than byte counts.
385
386<p>In the future the server will be able to correlate the bitmap with more
387detailed object data, so enough information is provided to associate the
388bitmap data with virtual addresses.
389
390<p>Heaps may be broken into segments within the VM, and due to memory
391constraints it may be desirable to send the bitmap in smaller pieces,
392so the protocol allows the heap data to be sent in several chunks.
393To avoid ambiguity, the client is required
394to send explicit "start" and "end" messages during an update.
395
396<p>All messages include a "heap ID" that can be used to differentiate
397between multiple independent virtual heaps or perhaps a native heap.  The
398client is allowed to send information about different heaps simultaneously,
399so all heap-specific information is tagged with a "heap ID".
400
401<h4>Chunk HPIF (server --&gt; client)</h4>
402<p>Request heap info.
403<pre>
404u1   when to send
405</pre>
406<p>The "when" values are:
407<pre>
4080: never
4091: immediately
4102: at the next GC
4113: at every GC
412</pre>
413
414<h4>Chunk HPIF (client --&gt; server, reply only)</h4>
415<p>Heap Info.  General information about the heap, suitable for a summary
416display.
417<pre>
418u4   number of heaps
419</pre>
420For each heap:
421<pre>
422u4   heap ID
423u8   timestamp in ms since Unix epoch
424u1   capture reason (same as 'when' value from server)
425u4   max heap size in bytes (-Xmx)
426u4   current heap size in bytes
427u4   current number of bytes allocated
428u4   current number of objects allocated
429</pre>
430<p>[We can get some of this from HPSG, more from HPSO.]
431<p>[Do we need a "heap overhead" stat here, indicating how much goes to
432waste?  e.g. (8 bytes per object * number of objects)]
433
434<h4>Chunk HPSG (server --&gt; client)</h4>
435<p>Request transmission of heap segment data.
436<pre>
437u1   when to send
438u1   what to send
439</pre>
440<p>The "when" to send will be zero to disable transmission, 1 to send
441during a GC.  Other values are currently undefined.  (Could use to pick
442which part of the GC to send it, or cause periodic transmissions.)
443<p>The "what" field is currently 0 for HPSG and 1 for HPSO.
444<p>No reply is expected.
445
446<h4>Chunk NHSG (server --&gt; client)</h4>
447<p>Request transmission of native heap segment data.
448<pre>
449u1   when to send
450u1   what to send
451</pre>
452<p>The "when" to send will be zero to disable transmission, 1 to send
453during a GC.  Other values are currently undefined.
454<p>The "what" field is currently ignored.
455<p>No reply is expected.
456
457<h4>Chunk HPST/NHST (client --&gt; server)</h4>
458<p>This is a Heap Start message.  It tells the server to discard any
459existing notion of what the client's heap looks like, and prepare for
460new information.  HPST indicates a virtual heap dump and must be followed
461by zero or more HPSG/HPSO messages and an HPEN.  NHST indicates a native
462heap dump and must be followed by zero or more NHSG messages and an NHEN.
463
464<p>The only data item is:
465<pre>
466u4   heap ID
467</pre>
468
469<h4>Chunk HPEN/NHEN (client --&gt; server)</h4>
470<p>Heap End, indicating that all information about the heap has been sent.
471A HPST will be paired with an HPEN and an NHST will be paired with an NHEN.
472
473<p>The only data item is:
474<pre>
475u4   heap ID
476</pre>
477
478<h4>Chunk HPSG (client --&gt; server)</h4>
479<p>Heap segment data.  Each chunk describes all or part of a contiguous
480stretch of heap memory.
481<pre>
482u4   heap ID
483u1   size of allocation unit, in bytes (e.g. 8 bytes)
484u4   virtual address of segment start
485u4   offset of this piece (relative to the virtual address)
486u4   length of piece, in allocation units
487var  usage data
488</pre>
489<p>The "usage data" indicates the status of each allocation unit.  The data
490is a stream of pairs of bytes, where the first byte indicates the state
491of the allocation unit, and the second byte indicates the number of
492consecutive allocation units with the same state.
493<p>The bits in the "state" byte have the following meaning:
494<pre>
495+---------------------------------------+
496|  7 |  6 |  5 |  4 |  3 |  2 |  1 |  0 |
497+---------------------------------------+
498|  P | U0 | K2 | K1 | K0 | S2 | S1 | S0 |
499+---------------------------------------+
500</pre>
501<ul>
502    <li>'S': solidity
503    <ul>
504        <li>0=free
505        <li>1=has hard reference
506        <li>2=has soft reference
507        <li>3=has weak reference
508        <li>4=has phantom reference
509        <li>5=pending finalization
510        <li>6=marked, about to be swept
511    </ul>
512    <li>'K': kind
513    <ul>
514        <li>0=object
515        <li>1=class object
516        <li>2=array of byte/boolean
517        <li>3=array of char/short
518        <li>4=array of Object/int/float
519        <li>5=array of long/double
520    </ul>
521    <li>'P': partial flag (not used for HPSG)
522    <li>'U': unused, must be zero
523</ul>
524
525<p>The use of the various 'S' types depends on when the information is
526sent.  The current plan is to send it either immediately after a GC,
527or between the "mark" and "sweep" phases of the GC.  For a fancy generational
528collector, we may just want to send it up periodically.
529
530<p>The run-length byte indicates the number of allocation units minus one, so a
531length of 255 means there are 256 consecutive units with this state.  In
532some cases, e.g. arrays of bytes, the actual size of the data is rounded
533up the nearest allocation unit.
534<p>For HPSG, the runs do not end at object boundaries.  It is not possible
535to tell from this bitmap whether a run contains one or several objects.
536(But see HPSO, below.)
537<p>[If we find that we have many long runs, we can overload the 'P' flag
538or dedicate the 'U' flag to indicate that we have a 16-bit length instead
539of 8-bit.  We can also use a variable-width integer scheme for the length,
540encoding 1-128 in one byte, 1-16384 in two bytes, etc.]
541<p>[Alternate plan for 'K': array of byte, array of char, array of Object,
542array of miscellaneous primitive type]
543<p>To parse the data, the server runs through the usage data until either
544(a) the end of the chunk is reached, or (b) all allocation units have been
545accounted for.  (If these two things don't happen at the same time, the
546chunk is rejected.)
547<p>Example: suppose a VM has a heap at 0x10000 that is 0x2000 bytes long
548(with an 8-byte allocation unit size, that's 0x0400 units long).
549The client could send one chunk (allocSize=8, virtAddr=0x10000, offset=0,
550length=0x0400) or two (allocSize=8, virtAddr=0x10000, offset=0, length=0x300;
551then allocSize=8, virtAddr=0x10000, offset=0x300, length=0x100).
552<p>The client must encode the entire heap, including all free space at
553the end, or the server will not have an accurate impression of the amount
554of memory in the heap.  This refers to the current heap size, not the
555maximum heap size.
556
557<h4>Chunk HPSO (client --&gt; server)</h4>
558<p>This is essentially identical to HPSG, but the runs are terminated at
559object boundaries.  If an object is larger than 256 allocation units, the
560"partial" flag is set in all runs except the last.
561<p>The resulting unpacked bitmap is identical, but the object boundary
562information can be used to gain insights into heap layout.
563<p>[Do we want to have a separate message for this?  Maybe just include
564a "variant" flag in the HPST packet.  Another possible form of output
565would be one that indicates the age, in generations, of each block of
566memory.  That would provide a quick visual indication of "permanent vs.
567transient residents", perhaps with a 16-level grey scale.]
568
569<h4>Chunk NHSG (client --&gt; server)</h4>
570<p>Native heap segment data.  Each chunk describes all or part of a
571contiguous stretch of native heap memory.  The format is the same as
572for HPSG, except that only solidity values 0 (= free) and 1 (= hard
573reference) are used, and the kind value is always 0 for free chunks
574and 7 for allocated chunks, indicating a non-VM object.
575<pre>
576u4   heap ID
577u1   size of allocation unit, in bytes (e.g. 8 bytes)
578u4   virtual address of segment start
579u4   offset of this piece (relative to the virtual address)
580u4   length of piece, in allocation units
581var  usage data
582</pre>
583
584<h3>Generic Replies</h3>
585
586The client-side chunk handlers need a common way to report simple success
587or failure.  By convention, an empty reply packet indicates success.
588
589<h4>Chunk FAIL (client --&gt; server, reply only)</h4>
590<p>The chunk includes a machine-readable error code and a
591human-readable error message.  Server code can associate the failure
592with the original request by comparing the JDWP packet ID.
593<p>This allows a standard way of, for example, rejecting badly-formed
594request packets.
595<pre>
596u4   error code
597u4   error message len (in 16-bit chars)
598var  error message (UTF-16)
599</pre>
600
601<h3>Miscellaneous</h3>
602
603<h4>Chunk EXIT (server --&gt; client)</h4>
604<p>Cause the client to exit with the specified status, using System.exit().
605Useful for certain kinds of testing.
606<pre>
607u4   exit status
608</pre>
609
610<h4>Chunk DTRC (server --&gt; client)</h4>
611<p>[TBD] start/stop dmtrace; can send the results back over the wire.  For
612size reasons we probably need "sending", "data", "key", "finished" as
6134 separate chunks/packets rather than one glob.
614
615
616<h2>Client API</h2>
617
618<p>The API is written in the Java programming language
619for convenience.  The code is free to call native methods if appropriate.
620
621<h3>Chunk Handler API</h3>
622
623<p>The basic idea is that arbitrary code can register handlers for
624specific chunk types.  When a DDM chunk with that type arrives, the
625appropriate handler is invoked.  The handler's return value provides the
626response to the server.
627
628<p>There are two packages.  android.ddm lives in the "framework" library,
629and has all of the chunk handlers and registration code.  It can freely
630use Android classes.  org.apache.harmony.dalvik.ddmc lives in the "core"
631library, and has
632some base classes and features that interact with the VM.  Nothing should
633need to modify the org.apache.harmony.dalvik.ddmc classes.
634
635<p>The DDM classes pass chunks of data around with a simple class:
636
637<pre class=prettyprint>
638class Chunk {
639    int type;
640    byte[] data;
641    int offset, length;
642};
643</pre>
644
645<p>The chunk handlers accept and return them:
646<pre class=prettyprint>
647public Chunk handleChunk(Chunk request)
648</pre>
649<p>The code is free to parse the chunk and generate a response in any
650way it chooses.  Big-endian byte ordering is recommended but not mandatory.
651<p>Chunk handlers will be notified when a DDM server connects or disconnects,
652so that they can perform setup and cleanup operations:
653<pre class=prettyprint>
654public void connected()
655public void disconnected()
656</pre>
657
658<p>The method processes the request, formulates a response, and returns it.
659If the method returns null, an empty JDWP success message will be returned.
660<p>The request/response interaction is essentially asynchronous in the
661protocol.  The packets are linked together with the JDWP message ID.
662<p>[We could use ByteBuffer here instead of byte[], but it doesn't gain
663us much.  Wrapping a ByteBuffer around an array is easy.  We don't want
664to pass the full packet in because we could have multiple chunks in one
665request packet.  The DDM code needs to collect and aggregate the responses
666to all chunks into a single JDWP response packet.  Parties wanting to
667write multiple chunks in response to a single chunk should send a null
668response back and use "sendChunk()" to send the data independently.]
669
670<h3>Unsolicited event API</h3>
671
672<p>If a piece of code wants to send a chunk of data to the server at some
673arbitrary time, it may do so with a method provided by
674org.apache.harmony.dalvik.DdmServer:
675
676<pre class=prettyprint>
677public static void sendChunk(Chunk chunk)
678</pre>
679
680<p>There is no response or status code.  No exceptions are thrown.
681
682
683<h2>Server API</h2>
684
685<p>This is similar to the client side in many ways, but makes extensive
686use of ByteBuffer in a perhaps misguided attempt to use java.nio.channels
687and avoid excessive thread creation and unnecessary data copying.
688
689<p>Upon receipt of a packet, the server will identify it as one of:
690<ol>
691    <li>Message to be passed through to the debugger
692    <li>Response to an earlier request
693    <li>Unsolicited event packet
694</ol>
695<p>To handle (2), when messages are sent from the server to the client,
696the message must be paired with a callback method.  The response might be
697delayed for a while -- or might never arrive -- so the server can't block
698waiting for responses from the client.
699<p>The chunk handlers look like this:
700<pre class=prettyprint>
701public void handleChunk(Client client, int type,
702    ByteBuffer data, boolean isReply, int msgId)
703</pre>
704<p>The arguments are:
705<dl>
706    <dt>client
707    <dd>An object representing the client VM that send us the packet.
708    <dt>type
709    <dd>The 32-bit chunk type.
710    <dt>data
711    <dd>The data.  The data's length can be determined by calling data.limit().
712    <dt>isReply
713    <dd>Set to "true" if this was a reply to a message we sent earlier,
714    "false" if the client sent this unsolicited.
715    <dt>msgId
716    <dd>The JDWP message ID.  Useful for connecting replies with requests.
717</dl>
718<p>If a handler doesn't like the contents of a packet, it should log an
719error message and return.  If the handler doesn't recognize the packet at
720all, it can call the superclass' handleUnknownChunk() method.
721
722<p>As with the client, the server code can be notified when clients
723connect or disconnect.  This allows the handler to send initialization
724code immediately after a connect, or clean up after a disconnect.
725<p>Data associated with a client can be stored in a ClientData object,
726which acts as a general per-client dumping around for VM and UI state.
727
728
729<P><BR>
730
731<HR>
732
733<address>Copyright &copy; 2007 The Android Open Source Project</address>
734
735</body>
736</HTML>
737