https://source.android.com/ 
/*
 * Copyright (C) 2006 The Android Open Source Project
 *
 * Licensed under the Apache License, Version 2.0 (the "License");
 * you may not use this file except in compliance with the License.
 * You may obtain a copy of the License at
 *
 *      http://www.apache.org/licenses/LICENSE-2.0
 *
 * Unless required by applicable law or agreed to in writing, software
 * distributed under the License is distributed on an "AS IS" BASIS,
 * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
 * See the License for the specific language governing permissions and
 * limitations under the License.
 */

package android.os;

import android.annotation.NonNull;
import android.annotation.Nullable;
import android.annotation.SystemApi;
import android.annotation.TestApi;
import android.util.Log;
import android.util.MutableInt;

import com.android.internal.annotations.GuardedBy;

import java.util.ArrayList;
import java.util.HashMap;



Gives access to the system properties store.  The system properties
store contains a list of string key-value pairs.
{@hide}

/**
 * Gives access to the system properties store.  The system properties
 * store contains a list of string key-value pairs.
 *
 * {@hide}
 */
@SystemApi
@TestApi
public class SystemProperties {
    private static final String TAG = "SystemProperties";
    private static final boolean TRACK_KEY_ACCESS = false;

    
Android O removed the property name length limit, but com.amazon.kindle 7.8.1.5
uses reflection to read this whenever text is selected (http://b/36095274).

@hide
/**
     * Android O removed the property name length limit, but com.amazon.kindle 7.8.1.5
     * uses reflection to read this whenever text is selected (http://b/36095274).
     * @hide
     */
    public static final int PROP_NAME_MAX = Integer.MAX_VALUE;

    
@hide
/** @hide */
    public static final int PROP_VALUE_MAX = 91;

    @GuardedBy("sChangeCallbacks")
    private static final ArrayList<Runnable> sChangeCallbacks = new ArrayList<Runnable>();

    @GuardedBy("sRoReads")
    private static final HashMap<String, MutableInt> sRoReads =
            TRACK_KEY_ACCESS ? new HashMap<>() : null;

    private static void onKeyAccess(String key) {
        if (!TRACK_KEY_ACCESS) return;

        if (key != null && key.startsWith("ro.")) {
            synchronized (sRoReads) {
                MutableInt numReads = sRoReads.getOrDefault(key, null);
                if (numReads == null) {
                    numReads = new MutableInt(0);
                    sRoReads.put(key, numReads);
                }
                numReads.value++;
                if (numReads.value > 3) {
                    Log.d(TAG, "Repeated read (count=" + numReads.value
                            + ") of a read-only system property '" + key + "'",
                            new Exception());
                }
            }
        }
    }

    private static native String native_get(String key);
    private static native String native_get(String key, String def);
    private static native int native_get_int(String key, int def);
    private static native long native_get_long(String key, long def);
    private static native boolean native_get_boolean(String key, boolean def);
    private static native void native_set(String key, String def);
    private static native void native_add_change_callback();
    private static native void native_report_sysprop_change();

    
Get the String value for the given key. 
Params:
  • key – the key to lookup
Returns:an empty string if the key isn't found
@hide
/**
     * Get the String value for the given {@code key}.
     *
     * @param key the key to lookup
     * @return an empty string if the {@code key} isn't found
     * @hide
     */
    @NonNull
    @SystemApi
    public static String get(@NonNull String key) {
        if (TRACK_KEY_ACCESS) onKeyAccess(key);
        return native_get(key);
    }

    
Get the String value for the given key. 
Params:
  • key – the key to lookup
  • def – the default value in case the property is not set or empty
Returns:if the key isn't found, return def if it isn't null, or an empty string otherwise
@hide
/**
     * Get the String value for the given {@code key}.
     *
     * @param key the key to lookup
     * @param def the default value in case the property is not set or empty
     * @return if the {@code key} isn't found, return {@code def} if it isn't null, or an empty
     * string otherwise
     * @hide
     */
    @NonNull
    @SystemApi
    @TestApi
    public static String get(@NonNull String key, @Nullable String def) {
        if (TRACK_KEY_ACCESS) onKeyAccess(key);
        return native_get(key, def);
    }

    
Get the value for the given key, and return as an integer. 
Params:
  • key – the key to lookup
  • def – a default value to return
Returns:the key parsed as an integer, or def if the key isn't found or
        cannot be parsed
@hide
/**
     * Get the value for the given {@code key}, and return as an integer.
     *
     * @param key the key to lookup
     * @param def a default value to return
     * @return the key parsed as an integer, or def if the key isn't found or
     *         cannot be parsed
     * @hide
     */
    @SystemApi
    public static int getInt(@NonNull String key, int def) {
        if (TRACK_KEY_ACCESS) onKeyAccess(key);
        return native_get_int(key, def);
    }

    
Get the value for the given key, and return as a long. 
Params:
  • key – the key to lookup
  • def – a default value to return
Returns:the key parsed as a long, or def if the key isn't found or
        cannot be parsed
@hide
/**
     * Get the value for the given {@code key}, and return as a long.
     *
     * @param key the key to lookup
     * @param def a default value to return
     * @return the key parsed as a long, or def if the key isn't found or
     *         cannot be parsed
     * @hide
     */
    @SystemApi
    public static long getLong(@NonNull String key, long def) {
        if (TRACK_KEY_ACCESS) onKeyAccess(key);
        return native_get_long(key, def);
    }

    
Get the value for the given key, returned as a boolean. Values 'n', 'no', '0', 'false' or 'off' are considered false. Values 'y', 'yes', '1', 'true' or 'on' are considered true. (case sensitive). If the key does not exist, or has any other value, then the default result is returned. 
Params:
  • key – the key to lookup
  • def – a default value to return
Returns:the key parsed as a boolean, or def if the key isn't found or is
        not able to be parsed as a boolean.
@hide
/**
     * Get the value for the given {@code key}, returned as a boolean.
     * Values 'n', 'no', '0', 'false' or 'off' are considered false.
     * Values 'y', 'yes', '1', 'true' or 'on' are considered true.
     * (case sensitive).
     * If the key does not exist, or has any other value, then the default
     * result is returned.
     *
     * @param key the key to lookup
     * @param def a default value to return
     * @return the key parsed as a boolean, or def if the key isn't found or is
     *         not able to be parsed as a boolean.
     * @hide
     */
    @SystemApi
    public static boolean getBoolean(@NonNull String key, boolean def) {
        if (TRACK_KEY_ACCESS) onKeyAccess(key);
        return native_get_boolean(key, def);
    }

    
Set the value for the given key to val. 
Throws:
@hide
/**
     * Set the value for the given {@code key} to {@code val}.
     *
     * @throws IllegalArgumentException if the {@code val} exceeds 91 characters
     * @hide
     */
    public static void set(@NonNull String key, @Nullable String val) {
        if (val != null && !val.startsWith("ro.") && val.length() > PROP_VALUE_MAX) {
            throw new IllegalArgumentException("value of system property '" + key
                    + "' is longer than " + PROP_VALUE_MAX + " characters: " + val);
        }
        if (TRACK_KEY_ACCESS) onKeyAccess(key);
        native_set(key, val);
    }

    
Add a callback that will be run whenever any system property changes.

Params:
  • callback – The Runnable that should be executed when a system property changes.
@hide
/**
     * Add a callback that will be run whenever any system property changes.
     *
     * @param callback The {@link Runnable} that should be executed when a system property
     * changes.
     * @hide
     */
    public static void addChangeCallback(@NonNull Runnable callback) {
        synchronized (sChangeCallbacks) {
            if (sChangeCallbacks.size() == 0) {
                native_add_change_callback();
            }
            sChangeCallbacks.add(callback);
        }
    }

    @SuppressWarnings("unused")  // Called from native code.
    private static void callChangeCallbacks() {
        synchronized (sChangeCallbacks) {
            //Log.i("foo", "Calling " + sChangeCallbacks.size() + " change callbacks!");
            if (sChangeCallbacks.size() == 0) {
                return;
            }
            ArrayList<Runnable> callbacks = new ArrayList<Runnable>(sChangeCallbacks);
            for (int i=0; i<callbacks.size(); i++) {
                try {
                    callbacks.get(i).run();
                } catch (Throwable t) {
                    Log.wtf(TAG, "Exception in SystemProperties change callback", t);
                    // Ignore and try to go on.
                }
            }
        }
    }

    
Notifies listeners that a system property has changed

@hide
/**
     * Notifies listeners that a system property has changed
     * @hide
     */
    public static void reportSyspropChanged() {
        native_report_sysprop_change();
    }

    private SystemProperties() {
    }
}