概述
当我们使用广播Broadcast的时候，需要考虑程序安全问题。
- 我们发送的广播会不会被别的app接收到，导致数据泄漏。
- 别的恶意软件会不会发送伪造广播导致我们的程序收到广播后执行某些非法动作。
其实这两个问题可以使用Android源码中的LocalBroadcastManager来解决，LocalBroadcastManager是只在当前进程内进行通信，而用法与Broadcast相同。
LocalBroadcastManager与Broadcast比较有三个优点：
- 广播数据不会超出你的应用程序内部，不会别的程序接收到，所以不需要担心泄露隐私数据。
- 广播接收器不会接收到其他应用程序发送的广播，所以不必担心可以利用安全漏洞。
- 通信方式比Broadcast更加高效，LocalBroadcastManager使用handler为核心机制，Broadcast则是使用Android内部的Binder机制。
基本使用
LocalBroadcastManager与Broadcast的使用方式完全一致，基本用法就是注册、反注册、发送广播。但是，LocalBroadcastManager只适用于动态注册使用，在AndroidManifest中注册的广播接收，则不适用。
LocalBroadcastManager lbm;
BroadcastReceiver receiver;
@Override
protected void onCreate(Bundle savedInstanceState) {
super.onCreate(savedInstanceState);
setContentView(R.layout.activity_main);
//注册
receiver = new BroadcastReceiver() {
@Override
public void onReceive(Context context, Intent intent) {
Log.i(TAG, "LocalBroadcastManager onReceive");
//do samething...
}
};
lbm = LocalBroadcastManager.getInstance(this);
lbm.registerReceiver(receiver, new IntentFilter("test_action"));
}
@Override
protected void onDestroy() {
super.onDestroy();
//反注册
lbm.unregisterReceiver(receiver);
}
@Override
public void onClick(View v) {
if(v == btnSendBroadcast){
//发送广播
lbm.sendBroadcast(new Intent("test_action"));
}
}
源码分析
构造方法
LocalBroadcastManager使用单例模式构造，在构造方法内部创建一个Handler，Handler用来执行广播的分发。
static final int MSG_EXEC_PENDING_BROADCASTS = 1;
//上下文对象
private final Context mAppContext;
//LocalBroadcastManager的核心机制是Handler
private final Handler mHandler;
//同步锁
private static final Object mLock = new Object();
//LocalBroadcastManager实例对象
private static LocalBroadcastManager mInstance;
//LocalBroadcastManager采用的单例模式
public static LocalBroadcastManager getInstance(Context context) {
synchronized (mLock) {
if (mInstance == null) {
//上下文对象为getApplicationContext，不用担心内存泄漏问题
mInstance = new LocalBroadcastManager(context.getApplicationContext());
}
return mInstance;
}
}
//私有构造函数
private LocalBroadcastManager(Context context) {
mAppContext = context;
//传入context.getMainLooper()，Handler执行在主线程
mHandler = new Handler(context.getMainLooper()) {
@Override
public void handleMessage(Message msg) {
switch (msg.what) {
case MSG_EXEC_PENDING_BROADCASTS:
executePendingBroadcasts();
break;
default:
super.handleMessage(msg);
}
}
};
}
基本数据结构和内部类
- 内部类ReceiverRecord，用于存放相对应的IntentFilter和BroadcastReceiver。
- 内部类BroadcastRecord，用于存放相对应的Intent和ReceiverRecord。
//存放相对应的IntentFilter和BroadcastReceiver
private static class ReceiverRecord {
final IntentFilter filter;
final BroadcastReceiver receiver;
boolean broadcasting;
ReceiverRecord(IntentFilter _filter, BroadcastReceiver _receiver) {
filter = _filter;
receiver = _receiver;
}
@Override
public String toString() {
StringBuilder builder = new StringBuilder(128);
builder.append("Receiver{");
builder.append(receiver);
builder.append(" filter=");
builder.append(filter);
builder.append("}");
return builder.toString();
}
}
//存放相对应的Intent和ReceiverRecord
private static class BroadcastRecord {
final Intent intent;
final ArrayList<ReceiverRecord> receivers;
BroadcastRecord(Intent _intent, ArrayList<ReceiverRecord> _receivers) {
intent = _intent;
receivers = _receivers;
}
}
- HashMap mReceivers 存储广播和过滤器信息，方便管理BroadcastReceiver的注册和取消。
- HashMap mActions存储action和相对应的ReceiverRecord列表，方便管理广播Actions的分发和执行。
- mPendingBroadcasts 存储所有的BroadcastRecord，方便管理BroadcastRecord创建和销毁。
//方便管理BroadcastReceiver的注册和取消
private final HashMap<BroadcastReceiver, ArrayList<IntentFilter>> mReceivers
= new HashMap<BroadcastReceiver, ArrayList<IntentFilter>>();
//方便管理广播Actions的分发和执行
private final HashMap<String, ArrayList<ReceiverRecord>> mActions
= new HashMap<String, ArrayList<ReceiverRecord>>();
//管理BroadcastRecord，方便执行
private final ArrayList<BroadcastRecord> mPendingBroadcasts
= new ArrayList<BroadcastRecord>();
注册与反注册
LocalBroadcastManager的注册与反注册，直白的说就是将广播相对应的必要元素（BroadcastReceiver、IntentFilter、Action）进行添加和移除。
注册广播：
/**
* Register a receive for any local broadcasts that match the given IntentFilter.
*
* @param receiver The BroadcastReceiver to handle the broadcast.
* @param filter Selects the Intent broadcasts to be received.
*
* @see #unregisterReceiver
*/
public void registerReceiver(BroadcastReceiver receiver, IntentFilter filter) {
synchronized (mReceivers) {
ReceiverRecord entry = new ReceiverRecord(filter, receiver);
ArrayList<IntentFilter> filters = mReceivers.get(receiver);
if (filters == null) {
filters = new ArrayList<IntentFilter>(1);
mReceivers.put(receiver, filters);
}
filters.add(filter);
for (int i=0; i<filter.countActions(); i++) {
String action = filter.getAction(i);
ArrayList<ReceiverRecord> entries = mActions.get(action);
if (entries == null) {
entries = new ArrayList<ReceiverRecord>(1);
mActions.put(action, entries);
}
entries.add(entry);
}
}
}
这里的注册主要就是将注册的广播接收器BroadcastReceiver和过滤器IntentFilter添加到相应的数据结构当中。将BroadcastReceiver作为key，把IntentFilter添加到对应的Value的集合中。将IntentFilter中的action作为key，将ReceiverRecord对象（包含对应的BroadcastReceiver和IntentFilter）添加到对应的Value的集合中。添加到这些数据结构中就是为了方便反注册和广播分发。
取消注册：
/**
* Unregister a previously registered BroadcastReceiver. <em>All</em>
* filters that have been registered for this BroadcastReceiver will be
* removed.
*
* @param receiver The BroadcastReceiver to unregister.
*
* @see #registerReceiver
*/
public void unregisterReceiver(BroadcastReceiver receiver) {
synchronized (mReceivers) {
ArrayList<IntentFilter> filters = mReceivers.remove(receiver);
if (filters == null) {
return;
}
for (int i=0; i<filters.size(); i++) {
IntentFilter filter = filters.get(i);
for (int j=0; j<filter.countActions(); j++) {
String action = filter.getAction(j);
ArrayList<ReceiverRecord> receivers = mActions.get(action);
if (receivers != null) {
for (int k=0; k<receivers.size(); k++) {
if (receivers.get(k).receiver == receiver) {
receivers.remove(k);
k--;
}
}
if (receivers.size() <= 0) {
mActions.remove(action);
}
}
}
}
}
}
取消注册就是将需要取消的BroadcastReceiver以及对应的IntentFilter和Action全部移除。源码中使用了3个for循环，遍历了mReceivers和mActions，移除所有的相应元素。
发送与分发
发送广播：
/**
* Broadcast the given intent to all interested BroadcastReceivers. This
* call is asynchronous; it returns immediately, and you will continue
* executing while the receivers are run.
*
* @param intent The Intent to broadcast; all receivers matching this
* Intent will receive the broadcast.
*
* @see #registerReceiver
*/
public boolean sendBroadcast(Intent intent) {
synchronized (mReceivers) {
final String action = intent.getAction();
final String type = intent.resolveTypeIfNeeded(
mAppContext.getContentResolver());
final Uri data = intent.getData();
final String scheme = intent.getScheme();
final Set<String> categories = intent.getCategories();
final boolean debug = DEBUG ||
((intent.getFlags() & Intent.FLAG_DEBUG_LOG_RESOLUTION) != 0);
if (debug) Log.v(
TAG, "Resolving type " + type + " scheme " + scheme
+ " of intent " + intent);
ArrayList<ReceiverRecord> entries = mActions.get(intent.getAction());
if (entries != null) {
if (debug) Log.v(TAG, "Action list: " + entries);
ArrayList<ReceiverRecord> receivers = null;
for (int i=0; i<entries.size(); i++) {
ReceiverRecord receiver = entries.get(i);
if (debug) Log.v(TAG, "Matching against filter " + receiver.filter);
if (receiver.broadcasting) {
if (debug) {
Log.v(TAG, " Filter's target already added");
}
continue;
}
int match = receiver.filter.match(action, type, scheme, data,
categories, "LocalBroadcastManager");
if (match >= 0) {
if (debug) Log.v(TAG, " Filter matched! match=0x" +
Integer.toHexString(match));
if (receivers == null) {
receivers = new ArrayList<ReceiverRecord>();
}
receivers.add(receiver);
receiver.broadcasting = true;
} else {
if (debug) {
String reason;
switch (match) {
case IntentFilter.NO_MATCH_ACTION: reason = "action"; break;
case IntentFilter.NO_MATCH_CATEGORY: reason = "category"; break;
case IntentFilter.NO_MATCH_DATA: reason = "data"; break;
case IntentFilter.NO_MATCH_TYPE: reason = "type"; break;
default: reason = "unknown reason"; break;
}
Log.v(TAG, " Filter did not match: " + reason);
}
}
}
if (receivers != null) {
for (int i=0; i<receivers.size(); i++) {
receivers.get(i).broadcasting = false;
}
mPendingBroadcasts.add(new BroadcastRecord(intent, receivers));
if (!mHandler.hasMessages(MSG_EXEC_PENDING_BROADCASTS)) {
mHandler.sendEmptyMessage(MSG_EXEC_PENDING_BROADCASTS);
}
return true;
}
}
}
return false;
}
发送广播的内部机制简单来说就是，根据Intent中的action，从mAction中取出对应的ReceiverRecord列表，然后遍历所有的ReceiverRecord，判断其IntentFilter是否与Intent中的action, type, scheme, data, categories相匹配，如果匹配成功，就将ReceiverRecord添加到mPendingBroadcasts中，最后执行mHandler.sendEmptyMessage(int what)交由Handler去执行。
分发执行：
private void executePendingBroadcasts() {
while (true) {
BroadcastRecord[] brs = null;
synchronized (mReceivers) {
final int N = mPendingBroadcasts.size();
if (N <= 0) {
return;
}
brs = new BroadcastRecord[N];
mPendingBroadcasts.toArray(brs);
mPendingBroadcasts.clear();
}
for (int i=0; i<brs.length; i++) {
BroadcastRecord br = brs[i];
for (int j=0; j<br.receivers.size(); j++) {
br.receivers.get(j).receiver.onReceive(mAppContext, br.intent);
}
}
}
}
mHandler收到消息后会执行executePendingBroadcasts()方法，将mPendingBroadcasts
转换成数组brs，遍历brs中所有的BroadcastRecord，再遍历所有的BroadcastReceiver，调用其onReceive方法，完成整个广播的执行。
总结
在整个源码分析的过程中，我们可以清晰的看到，LocalBroadcastManager的核心机制还是使用的Handler，与一般的广播使用Binder机制完全不一样。从而做到了开篇所说的几个优点：当前APP内部通信，数据安全，无可利用漏洞，且效率更高。
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