1<html> 2<head> 3<title>Dalvik Debugger Support</title> 4</head> 5 6<body> 7<h1>Dalvik Debugger Support</h1> 8 9<p> 10The Dalvik virtual machine supports source-level debugging with many popular 11development environments. Any tool that allows remote debugging over JDWP 12(the 13<a href="http://java.sun.com/javase/6/docs/technotes/guides/jpda/jdwp-spec.html"> 14Java Debug Wire Protocol</a>) is expected work. Supported debuggers 15include jdb, Eclipse, IntelliJ, and JSwat. 16</p><p> 17The VM does not support tools based on JVMTI (Java Virtual 18Machine Tool Interface). This is a relatively intrusive approach that 19relies on bytecode insertion, something the Dalvik VM does not currently 20support. 21</p><p> 22Dalvik's implementation of JDWP also includes hooks for supporting 23DDM (Dalvik Debug Monitor) features, notably as implemented by DDMS 24(Dalvik Debug Monitor Server) and the Eclipse ADT plugin. The protocol 25and VM interaction is described in some detail 26<a href="debugmon.html">here</a>. 27</p><p> 28All of the debugger support in the VM lives in the <code>dalvik/vm/jdwp</code> 29directory, and is almost entirely isolated from the rest of the VM sources. 30<code>dalvik/vm/Debugger.c</code> bridges the gap. The goal in doing so 31was to make it easier to re-use the JDWP code in other projects. 32</p><p> 33 34 35<h2>Implementation</h2> 36 37<p> 38Every VM that has debugging enabled starts a "JDWP" thread. The thread 39typically sits idle until DDMS or a debugger connects. The thread is 40only responsible for handling requests from the debugger; VM-initated 41communication, such as notifying the debugger when the VM has stopped at 42a breakpoint, are sent from the affected thread. 43</p><p> 44When the VM is started from the Android app framework, debugging is enabled 45for all applications when the system property <code>ro.debuggable</code> 46is set to </code>1</code> (use <code>adb shell getprop ro.debuggable</code> 47to check it). If it's zero, debugging can be enabled via the application's 48manifest, which must include <code>android:debuggable="true"</code> in the 49<code><application></code> element. 50 51</p><p> 52The VM recognizes the difference between a connection from DDMS and a 53connection from a debugger (either directly or in concert with DDMS). 54A connection from DDMS alone doesn't result in a change in VM behavior, 55but when the VM sees debugger packets it allocates additional data 56structures and may switch to a different implementation of the interpreter. 57</p><p> 58Pre-Froyo implementations of the Dalvik VM used read-only memory mappings 59for all bytecode, which made it necessary to scan for breakpoints by 60comparing the program counter to a set of addresses. In Froyo this was 61changed to allow insertion of breakpoint opcodes. This allows the VM 62to execute code more quickly, and does away with the hardcoded limit 63of 20 breakpoints. Even with this change, however, the debug-enabled 64interpreter is much slower than the regular interpreter (perhaps 5x). 65</p><p> 66The JDWP protocol is stateless, so the VM handles individual debugger 67requests as they arrive, and posts events to the debugger as they happen. 68</p><p> 69 70 71<h2>Debug Data</h2> 72<p> Source code debug data, which includes mappings of source code to 73bytecode and lists describing which registers are used to hold method 74arguments and local variables, are optionally emitted by the Java compiler. 75When <code>dx</code> converts Java bytecode to Dalvik bytecode, it must 76also convert this debug data. 77</p><p> 78<code>dx</code> must also ensure that it doesn't perform operations 79that confuse the debugger. For example, re-using registers that hold 80method arguments and the "<code>this</code>" pointer is allowed in 81Dalvik bytecode if the values are never used or no longer needed. 82This can be very confusing for the debugger (and the programmer) 83since the values have method scope and aren't expected to disappear. For 84this reason, <code>dx</code> generates sub-optimal code in some situations 85when debugging support is enabled. 86</p><p> 87Some of the debug data is used for other purposes; in particular, having 88filename and line number data is necessary for generating useful exception 89stack traces. This data can be omitted by <code>dx</code> to make the DEX 90file smaller. 91</p><p> 92 93 94<h2>Usage</h2> 95 96<p> 97The Dalvik VM supports many of the same command-line flags that other popular 98desktop VMs do. To start a VM with debugging enabled, you add a command-line 99flag with some basic options. The basic incantation looks something 100like this: 101 102<pre>-Xrunjdwp:transport=dt_socket,address=8000,server=y,suspend=y</pre> 103or 104<pre>-agentlib:jdwp=transport=dt_socket,address=8000,server=y,suspend=y</pre> 105 106</p><p> 107After the initial prefix, options are provided as name=value pairs. The 108options currently supported by the Dalvik VM are: 109<dl> 110 <dt>transport (no default)</dt> 111 <dd>Communication transport mechanism to use. Dalvik supports 112 TCP/IP sockets (<code>dt_socket</code>) and connection over USB 113 through ADB (<code>dt_android_adb</code>). 114 </dd> 115 116 <dt>server (default='n')</dt> 117 <dd>Determines whether the VM acts as a client or a server. When 118 acting as a server, the VM waits for a debugger to connect to it. 119 When acting as a client, the VM attempts to connect to a waiting 120 debugger. 121 </dd> 122 123 <dt>suspend (default='n')</dt> 124 <dd>If set to 'y', the VM will wait for a debugger connection 125 before executing application code. When the debugger connects (or 126 when the VM finishes connecting to the debugger), the VM tells the 127 debugger that it has suspended, and will not proceed until told 128 to resume. If set to 'n', the VM just plows ahead. 129 </dd> 130 131 <dt>address (default="")</dt> 132 <dd>This must be <code>hostname:port</code> when <code>server=n</code>, 133 but can be just <code>port</code> when <code>server=y</code>. This 134 specifies the IP address and port number to connect or listen to. 135 <br> 136 Listening on port 0 has a special meaning: try to 137 listen on port 8000; if that fails, try 8001, 8002, and so on. (This 138 behavior is non-standard and may be removed from a future release.) 139 <br>This option has no meaning for <code>transport=dt_android_adb</code>. 140 </dd> 141 142 <dt>help (no arguments)</dt> 143 <dd>If this is the only option, a brief usage message is displayed. 144 </dd> 145 146 <dt>launch, onthrow, oncaught, timeout</dt> 147 <dd>These options are accepted but ignored. 148 </dd> 149</dl> 150 151</p><p> 152To debug a program on an Android device using DDMS over USB, you could 153use a command like this: 154<pre>% dalvikvm -agentlib:jdwp=transport=dt_android_adb,suspend=y,server=y -cp /data/foo.jar Foo</pre> 155 156This tells the Dalvik VM to run the program with debugging enabled, listening 157for a connection from DDMS, and waiting for a debugger. The program will show 158up with an app name of "?" in the process list, because it wasn't started 159from the Android application framework. From here you would connect your 160debugger to the appropriate DDMS listen port (e.g. 161<code>jdb -attach localhost:8700</code> after selecting it in the app list). 162 163</p><p> 164To debug a program on an Android device using TCP/IP bridged across ADB, 165you would first need to set up forwarding: 166<pre>% adb forward tcp:8000 tcp:8000 167% adb shell dalvikvm -agentlib:jdwp=transport=dt_socket,address=8000,suspend=y,server=y -cp /data/foo.jar Foo</pre> 168and then <code>jdb -attach localhost:8000</code>. 169</p><p> 170(In the above examples, the VM will be suspended when you attach. In jdb, 171type <code>cont</code> to continue.) 172</p><p> 173The DDMS integration makes the <code>dt_android_adb</code> transport much 174more convenient when debugging on an Android device, but when working with 175Dalvik on the desktop it makes sense to use the TCP/IP transport. 176</p><p> 177 178 179<h2>Known Issues and Limitations</h2> 180 181</p><p> 182Most of the optional features JDWP allows are not implemented. These 183include field access watchpoints and better tracking of monitors. 184</p><p> 185Not all JDWP requests are implemented. In particular, anything that 186never gets emitted by the debuggers we've used is not supported and will 187result in error messages being logged. Support will be added when a 188use case is uncovered. 189</p><p> 190 191</p><p> 192The debugger and garbage collector are somewhat loosely 193integrated at present. The VM currently guarantees that any object the 194debugger is aware of will not be garbage collected until after the 195debugger disconnects. This can result in a build-up over time while the 196debugger is connected. For example, if the debugger sees a running 197thread, the associated Thread object will not be collected, even after 198the thread terminates. 199</p><p> 200The only way to "unlock" the references is to detach and reattach the 201debugger. 202</p><p> 203 204</p><p> 205The translation from Java bytecode to Dalvik bytecode may result in 206identical sequences of instructions being combined. This can make it 207look like the wrong bit of code is being executed. For example: 208<pre> int test(int i) { 209 if (i == 1) { 210 return 0; 211 } 212 return 1; 213 }</pre> 214The Dalvik bytecode uses a common <code>return</code> instruction for both 215<code>return</code> statements, so when <code>i</code> is 1 the debugger 216will single-step through <code>return 0</code> and then <code>return 1</code>. 217</p><p> 218 219</p><p> 220Dalvik handles synchronized methods differently from other VMs. 221Instead of marking a method as <code>synchronized</code> and expecting 222the VM to handle the locks, <code>dx</code> inserts a "lock" 223instruction at the top of the method and an "unlock" instruction in a 224synthetic <code>finally</code> block. As a result, when single-stepping 225a <code>return</code> statement, the "current line" cursor may jump to 226the last line in the method. 227</p><p> 228This can also affect the way the debugger processes exceptions. The 229debugger may decide to break on an 230exception based on whether that exception is "caught" or "uncaught". To 231be considered uncaught, there must be no matching <code>catch</code> block 232or <code>finally</code> clause between the current point of execution and 233the top of the thread. An exception thrown within or below a synchronized 234method will always be considered "caught", so the debugger won't stop 235until the exception is re-thrown from the synthetic <code>finally</code> block. 236</p><p> 237 238 239<address>Copyright © 2009 The Android Open Source Project</address> 240</p> 241 242</body> 243</html> 244