How to define and use a system property in Android

Question

I am trying to use some arguments for an Instrumentation test. I noticed that I can read system properties with System.getProperty() function. So I use setprop command to set a system property. For example: adb shell setprop AP 123. Inside my Test code I try to read this AP property with :

tmp = System.getProperty("AP"); 
Log.d("MyTest","AP Value = " + tmp);

Then I use logcat to view this debug message but I get a null value for this property. Any ideas on what could be wrong? Note that I can still read the system property with adb shell getprop AP command.

Answer 1

Based on provided answer, Slightly modified version of SetProperty

    public void setSystemProperty(String Key, String value){
    InputStreamReader in = null;
    BufferedReader reader = null;
    try {
        Process proc = Runtime.getRuntime().exec("/system/bin/setprop "+Key+" "+value);
        in = new InputStreamReader(proc.getInputStream());
        reader = new BufferedReader(in);

        String line = null;
        Log.d("Saurabh Shell" ,"<OUTPUT>");
        while ( (line = reader.readLine()) != null)
            Log.d("Shell" , line);
        Log.d("Saurabh Shell", "</OUTPUT>");
        int exitVal = proc.waitFor();
        Log.d("Saurabh Shell","Process exitValue: " + exitVal);

    } catch (IOException e) {
       e.printStackTrace();
    } catch (InterruptedException e) {
        e.printStackTrace();
    } finally {
        closeQuietly(in);
        closeQuietly(reader);
    }
}

close Input and reader

    public  void closeQuietly(Closeable closeable) {
    if (closeable == null) return;
    try {
        closeable.close();
    } catch (IOException ignored) {
    }
}

Answer 2

System properties are read once when the root VM (Zygote) is started, which in turn spawns other Dalvik VMs like that of your application. That means you cannot set system properties on the fly.

Try restarting Zygote using adb shell stop (wait until it has stopped) and adb shell start (wait until it has restarted), then try again. Or simply reboot the device or emulator.

Answer 3

Because there are two types of property in Android.

1. System level - we can get/set with command adb shell getprop/setprop.
2. In current process level - we can get/set with regular java System.getProperty()/setProperty().

As you are setting a system level property and trying to get its value as current process level, you are getting null value in log.

Answer 4

import android.os.SystemProperties

String s = SystemProterties.get("ro.xxx.xxx","default value if property not set");

Answer 5

here's a slightly saner version based on accuya's answer:

public static String readSystemProperty(String name) {
    InputStreamReader in = null;
    BufferedReader reader = null;
    try {
        Process proc = Runtime.getRuntime().exec(new String[]{"/system/bin/getprop", name});
        in = new InputStreamReader(proc.getInputStream());
        reader = new BufferedReader(in);
        return reader.readLine();
    } catch (IOException e) {
        return null;
    } finally {
        closeQuietly(in);
        closeQuietly(reader);
    }
}

public static void closeQuietly(Closeable closeable) {
    if (closeable == null) return;
    try {
        closeable.close();
    } catch (IOException ignored) {
    }
}

Answer 6

To get the property set by 'setprop', there are two options:
One. use android.os.SystemProperties, this is a hide API. use it like this:

Class clazz = null;
clazz = Class.forName("android.os.SystemProperties");
Method method = clazz.getDeclaredMethod("get", String.class);
String prop = (String)method.invoke(null, "AP");
Log.e("so_test", "my prop is: <" + prop  + ">");

Two. use 'getprop' utility:

Process proc = Runtime.getRuntime().exec(new String[]{"/system/bin/getprop", "AP"});
BufferedReader reader = new BufferedReader(new InputStreamReader(proc.getInputStream()));
Log.e("so_test", "my prop is: " + reader.readLine());

Maybe using functions availble in NDK is an option too, but why bother?