How DDMS works

DDMS acts as a middleman to connect the IDE to the applications running on the device. On Android, every application runs in its own process, each of which hosts its own virtual machine (VM). And each process listens for a debugger on a different port.

When it starts, DDMS connects to adb and starts a device monitoring service between the two, which will notify DDMS when a device is connected or disconnected. When a device is connected, a VM monitoring service is created between adb and DDMS, which will notify DDMS when a VM on the device is started or terminated. Once a VM is running, DDMS retrieves the the VM's process ID (pid), via adb, and opens a connection to the VM's debugger, through the adb daemon (adbd) on the device. DDMS can now talk to the VM using a custom wire protocol.

For each VM on the device, DDMS opens a port upon which it will listen for a debugger. For the first VM, DDMS listens for a debugger on port 8600, the next on 8601, and so on. When a debugger connects to one of these ports, all traffic is forwarded between the debugger and the associated VM. Debugging can then process like any remote debugging session.

DDMS also opens another local port, the DDMS "base port" (8700, by default), upon which it also listens for a debugger. When a debugger connects to this base port, all traffic is forwarded to the VM currently selected in DDMS, so this is typically where you debugger should connect.

For more information on port-forwarding with DDMS, read Configuring your IDE to attach to port 8700 for debugging.

Tip: You can set a number of DDMS preferences in File > Preferences. Preferences are saved to "$HOME/.ddmsrc".

Known debugging issues with Dalvik

Debugging an application in the Dalvik VM should work the same as it does in other VMs. However, when single-stepping out of synchronized code, the "current line" cursor may jump to the last line in the method for one step.

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