The Dalvik virtual machine supports source-level debugging with many popular development environments. Any tool that allows remote debugging over JDWP (the Java Debug Wire Protocol) is expected work. Supported debuggers include jdb, Eclipse, IntelliJ, and JSwat.
The VM does not support tools based on JVMTI (Java Virtual Machine Tool Interface). This is a relatively intrusive approach that relies on bytecode insertion, something the Dalvik VM does not currently support.
Dalvik's implementation of JDWP also includes hooks for supporting DDM (Dalvik Debug Monitor) features, notably as implemented by DDMS (Dalvik Debug Monitor Server) and the Eclipse ADT plugin. The protocol and VM interaction is described in some detail here.
All of the debugger support in the VM lives in the dalvik/vm/jdwp
directory, and is almost entirely isolated from the rest of the VM sources.
dalvik/vm/Debugger.c bridges the gap. The goal in doing so
was to make it easier to re-use the JDWP code in other projects.
Every VM that has debugging enabled starts a "JDWP" thread. The thread typically sits idle until DDMS or a debugger connects. The thread is only responsible for handling requests from the debugger; VM-initated communication, such as notifying the debugger when the VM has stopped at a breakpoint, are sent from the affected thread.
When the VM is started from the Android app framework, debugging is enabled
for all applications when the system property ro.debuggable
is set to 1 (use adb shell getprop ro.debuggable
to check it). If it's zero, debugging can be enabled via the application's
manifest, which must include android:debuggable="true" in the
<application> element.
The VM recognizes the difference between a connection from DDMS and a connection from a debugger (either directly or in concert with DDMS). A connection from DDMS alone doesn't result in a change in VM behavior, but when the VM sees debugger packets it allocates additional data structures and may switch to a different implementation of the interpreter.
Pre-Froyo implementations of the Dalvik VM used read-only memory mappings for all bytecode, which made it necessary to scan for breakpoints by comparing the program counter to a set of addresses. In Froyo this was changed to allow insertion of breakpoint opcodes. This allows the VM to execute code more quickly, and does away with the hardcoded limit of 20 breakpoints. Even with this change, however, the debug-enabled interpreter is much slower than the regular interpreter (perhaps 5x).
The JDWP protocol is stateless, so the VM handles individual debugger requests as they arrive, and posts events to the debugger as they happen.
Source code debug data, which includes mappings of source code to
bytecode and lists describing which registers are used to hold method
arguments and local variables, are optionally emitted by the Java compiler.
When dx converts Java bytecode to Dalvik bytecode, it must
also convert this debug data.
dx must also ensure that it doesn't perform operations
that confuse the debugger. For example, re-using registers that hold
method arguments and the "this" pointer is allowed in
Dalvik bytecode if the values are never used or no longer needed.
This can be very confusing for the debugger (and the programmer)
since the values have method scope and aren't expected to disappear. For
this reason, dx generates sub-optimal code in some situations
when debugging support is enabled.
Some of the debug data is used for other purposes; in particular, having
filename and line number data is necessary for generating useful exception
stack traces. This data can be omitted by dx to make the DEX
file smaller.
The Dalvik VM supports many of the same command-line flags that other popular desktop VMs do. To start a VM with debugging enabled, you add a command-line flag with some basic options. The basic incantation looks something like this:
-Xrunjdwp:transport=dt_socket,address=8000,server=y,suspend=yor
-agentlib:jdwp=transport=dt_socket,address=8000,server=y,suspend=y
After the initial prefix, options are provided as name=value pairs. The options currently supported by the Dalvik VM are:
dt_socket) and connection over USB
through ADB (dt_android_adb).
hostname:port when server=n,
but can be just port when server=y. This
specifies the IP address and port number to connect or listen to.
transport=dt_android_adb.
To debug a program on an Android device using DDMS over USB, you could use a command like this:
% dalvikvm -agentlib:jdwp=transport=dt_android_adb,suspend=y,server=y -cp /data/foo.jar FooThis tells the Dalvik VM to run the program with debugging enabled, listening for a connection from DDMS, and waiting for a debugger. The program will show up with an app name of "?" in the process list, because it wasn't started from the Android application framework. From here you would connect your debugger to the appropriate DDMS listen port (e.g.
jdb -attach localhost:8700 after selecting it in the app list).
To debug a program on an Android device using TCP/IP bridged across ADB, you would first need to set up forwarding:
% adb forward tcp:8000 tcp:8000 % adb shell dalvikvm -agentlib:jdwp=transport=dt_socket,address=8000,suspend=y,server=y -cp /data/foo.jar Fooand then
jdb -attach localhost:8000.
(In the above examples, the VM will be suspended when you attach. In jdb,
type cont to continue.)
The DDMS integration makes the dt_android_adb transport much
more convenient when debugging on an Android device, but when working with
Dalvik on the desktop it makes sense to use the TCP/IP transport.
Most of the optional features JDWP allows are not implemented. These include field access watchpoints and better tracking of monitors.
Not all JDWP requests are implemented. In particular, anything that never gets emitted by the debuggers we've used is not supported and will result in error messages being logged. Support will be added when a use case is uncovered.
The debugger and garbage collector are somewhat loosely integrated at present. The VM currently guarantees that any object the debugger is aware of will not be garbage collected until after the debugger disconnects. This can result in a build-up over time while the debugger is connected. For example, if the debugger sees a running thread, the associated Thread object will not be collected, even after the thread terminates.
The only way to "unlock" the references is to detach and reattach the debugger.
The translation from Java bytecode to Dalvik bytecode may result in identical sequences of instructions being combined. This can make it look like the wrong bit of code is being executed. For example:
int test(int i) {
if (i == 1) {
return 0;
}
return 1;
}
The Dalvik bytecode uses a common return instruction for both
return statements, so when i is 1 the debugger
will single-step through return 0 and then return 1.
Dalvik handles synchronized methods differently from other VMs.
Instead of marking a method as synchronized and expecting
the VM to handle the locks, dx inserts a "lock"
instruction at the top of the method and an "unlock" instruction in a
synthetic finally block. As a result, when single-stepping
a return statement, the "current line" cursor may jump to
the last line in the method.
This can also affect the way the debugger processes exceptions. The
debugger may decide to break on an
exception based on whether that exception is "caught" or "uncaught". To
be considered uncaught, there must be no matching catch block
or finally clause between the current point of execution and
the top of the thread. An exception thrown within or below a synchronized
method will always be considered "caught", so the debugger won't stop
until the exception is re-thrown from the synthetic finally block.
Copyright © 2009 The Android Open Source Project