SharedPreferences是以键值对的方式保存数据的，数据存放在xml文件中。

获取SharedPreferences

  1. Context类的getSharedPreferences()方法。参数一：指定文件名称；参数二：指定操作模式，Android4.2之后只有MODE_PRIVATE一种模式。
  2. Activity类的getPreferences()方法。文件名称默认是当前Activity的类名，接受一操作模式参数。
  3. PreferenceManager类中的getDefaultSharedPerferences()方法。这是一个静态方法，接受一个Context参数，并自动使用当前应用程序的包名作为前缀命名文件名。

SharedPreferences的使用

SharedPreferences对象本身不支持存储和修改，只能获取数据，存储修改是通过SharedPreferences.edit()获取内部接口Editor对象实现。

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 1）写入数据：
     //步骤1：创建一个SharedPreferences对象
     SharedPreferences sharedPreferences= getSharedPreferences("data",Context.MODE_PRIVATE);
     //步骤2： 实例化SharedPreferences.Editor对象
     SharedPreferences.Editor editor = sharedPreferences.edit();
     //步骤3：将获取过来的值放入文件
     editor.putString("name", “Tom”);
     editor.putInt("age", 28);
     editor.putBoolean("marrid",false);
     //步骤4：提交               
     editor.commit();


 2）读取数据：
     SharedPreferences sharedPreferences= getSharedPreferences("data", Context .MODE_PRIVATE);
     String userId=sharedPreferences.getString("name","");
  
3）删除指定数据
     editor.remove("name");
     editor.commit();


4）清空数据
     editor.clear();
     editor.commit();

关键点

  1. 没有xml时会创建文件
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public SharedPreferences getSharedPreferences(String name, int mode) { 
	File file; 
	synchronized (ContextImpl.class) { 
		if (mSharedPrefsPaths == null) { 
			mSharedPrefsPaths = new ArrayMap<>(); 
		} 
		//先从mSharedPrefsPaths查询是否存在相应文件 
		file = mSharedPrefsPaths.get(name); 
		if (file == null) { 
		//如果文件不存在, 则创建新的文件，文件的路径是/data/data/package name/shared_prefs/ 
		file = getSharedPreferencesPath(name); 
		mSharedPrefsPaths.put(name, file); 
		} 
	} 
	return getSharedPreferences(file, mode); }
  1. SharedPreferences加载文件使用异步加载的方式。
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private void startLoadFromDisk() { 
	synchronized (this) { mLoaded = false; } 
	new Thread("SharedPreferencesImpl-load") { 
		public void run() { loadFromDisk(); } 
	}.start(); 
}
  1. 读文件的时候会等待一个锁，调用线程会被阻塞。因此，如果文件太大可能会导致ANR。
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public String getString(String key, @Nullable String defValue) {
    synchronized (this) {
        //检查是否加载完成
        awaitLoadedLocked();
        String v = (String)mMap.get(key);
        return v != null ? v : defValue;
    }
}

private void awaitLoadedLocked() {
    if (!mLoaded) {
        // Raise an explicit StrictMode onReadFromDisk for this
        // thread, since the real read will be in a different
        // thread and otherwise ignored by StrictMode.
        BlockGuard.getThreadPolicy().onReadFromDisk();
    }
    while (!mLoaded) {
        try {
            mLock.wait();
        } catch (InterruptedException unused) {
        }
    }
    if (mThrowable != null) {
        throw new IllegalStateException(mThrowable);
    }
}
  1. 写文件实际上会创建一个新的Map用于记录要修改的内容。
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public Editor edit() {
    synchronized (this) {
        awaitLoadedLocked(); 
    }
    return new EditorImpl(); 
}

public final class EditorImpl implements Editor {
    private final Map<String, Object> mModified = Maps.newHashMap();
    private boolean mClear = false;

    //插入数据
    public Editor putString(String key, @Nullable String value) {
        synchronized (this) {
            //插入数据, 先暂存到mModified对象
            mModified.put(key, value);
            return this;
        }
    }
    //移除数据
    public Editor remove(String key) {
        synchronized (this) {
            mModified.put(key, this);
            return this;
        }
    }

    //清空全部数据
    public Editor clear() {
        synchronized (this) {
            mClear = true;
            return this;
        }
    }
}
  1. commit和apply的差异
    commit行是isFromSyncCommit，直接将runnable run了起来。是同步的过程。
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public boolean commit() {
    //将数据更新到内存
    MemoryCommitResult mcr = commitToMemory();
    //将内存数据同步到文件
    SharedPreferencesImpl.this.enqueueDiskWrite(mcr, null);
    try {
        //进入等待状态, 直到写入文件的操作完成
        mcr.writtenToDiskLatch.await();
    } catch (InterruptedException e) {
        return false;
    }
    //通知监听则, 并在主线程回调onSharedPreferenceChanged()方法
    notifyListeners(mcr);
    // 返回文件操作的结果数据
    return mcr.writeToDiskResult;
}

private MemoryCommitResult commitToMemory() {
    MemoryCommitResult mcr = new MemoryCommitResult();
    synchronized (SharedPreferencesImpl.this) {
        if (mDiskWritesInFlight > 0) {
            mMap = new HashMap<String, Object>(mMap);
        }
        //这里就是要写到磁盘的map
        mcr.mapToWriteToDisk = mMap;
        mDiskWritesInFlight++;

        //是否有监听key改变的监听者
        boolean hasListeners = mListeners.size() > 0;
        if (hasListeners) {
            mcr.keysModified = new ArrayList<String>();
            mcr.listeners = new HashSet<OnSharedPreferenceChangeListener>(mListeners.keySet());
        }

        synchronized (this) {
            //当mClear为true, 则直接清空mMap
            if (mClear) {
                if (!mMap.isEmpty()) {
                    mcr.changesMade = true;
                    mMap.clear();
                }
                mClear = false;
            }

            for (Map.Entry<String, Object> e : mModified.entrySet()) {
                String k = e.getKey();
                Object v = e.getValue();
                //注意此处的this是个特殊值, 用于移除相应的key操作.
                if (v == this || v == null) {
                    if (!mMap.containsKey(k)) {
                        continue;
                    }
                    mMap.remove(k);
                } else {
                    if (mMap.containsKey(k)) {
                        Object existingValue = mMap.get(k);
                        if (existingValue != null && existingValue.equals(v)) {
                            continue;
                        }
                    }
                    mMap.put(k, v);
                }

                mcr.changesMade = true; // changesMade代表数据是否有改变
                if (hasListeners) {
                    mcr.keysModified.add(k); //记录发生改变的key
                }
            }
            mModified.clear(); //清空EditorImpl中的mModified数据
        }
    }
    return mcr;
}

private void enqueueDiskWrite(final MemoryCommitResult mcr,
                              final Runnable postWriteRunnable) {
    final Runnable writeToDiskRunnable = new Runnable() {
            public void run() {
                synchronized (mWritingToDiskLock) {
                    //执行文件写入操作
                    writeToFile(mcr);
                }
                synchronized (SharedPreferencesImpl.this) {
                    mDiskWritesInFlight--;
                }
                //此时postWriteRunnable为null不执行该方法
                if (postWriteRunnable != null) {
                    postWriteRunnable.run();
                }
            }
        };

    final boolean isFromSyncCommit = (postWriteRunnable == null);

    if (isFromSyncCommit) { //commit方法会进入该分支
        boolean wasEmpty = false;
        synchronized (SharedPreferencesImpl.this) {
            //commitToMemory过程会加1,则wasEmpty=true
            wasEmpty = mDiskWritesInFlight == 1;
        }
        if (wasEmpty) {
            //跳转到上面
            writeToDiskRunnable.run();
            return;
        }
    }
    //不执行该方法
    QueuedWork.singleThreadExecutor().execute(writeToDiskRunnable);
}

apply是异步的过程，实际使用HandlerThread去执行的。

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public void apply() {
    final long startTime = System.currentTimeMillis();

    final MemoryCommitResult mcr = commitToMemory();
    final Runnable awaitCommit = new Runnable() {
            @Override
            public void run() {
                try {
                    mcr.writtenToDiskLatch.await();
                } catch (InterruptedException ignored) {
                }
            }
        };

    QueuedWork.addFinisher(awaitCommit);

    Runnable postWriteRunnable = new Runnable() {
            @Override
            public void run() {
                awaitCommit.run();
                QueuedWork.removeFinisher(awaitCommit);
            }
        };

    SharedPreferencesImpl.this.enqueueDiskWrite(mcr, postWriteRunnable);

    // Okay to notify the listeners before it's hit disk
    // because the listeners should always get the same
    // SharedPreferences instance back, which has the
    // changes reflected in memory.
    notifyListeners(mcr);
}

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QueuedWork.singleThreadExecutor().execute(writeToDiskRunnable);
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public static void queue(Runnable work, boolean shouldDelay) {
    Handler handler = getHandler();

    synchronized (sLock) {
        sWork.add(work);

        if (shouldDelay && sCanDelay) {
            handler.sendEmptyMessageDelayed(QueuedWorkHandler.MSG_RUN, DELAY);
        } else {
            handler.sendEmptyMessage(QueuedWorkHandler.MSG_RUN);
        }
    }
}

private static Handler getHandler() {
    synchronized (sLock) {
        if (sHandler == null) {
            HandlerThread handlerThread = new HandlerThread("queued-work-looper",
                    Process.THREAD_PRIORITY_FOREGROUND);
            handlerThread.start();

            sHandler = new QueuedWorkHandler(handlerThread.getLooper());
        }
        return sHandler;
    }
}
  • apply() 没有返回值而 commit() 返回 boolean 表明修改是否提交成功
  • commit() 是把内容同步提交到硬盘的，而 apply() 先立即把修改提交到内存，然后开启一个异步的线程提交到硬盘，并且如果提交失败，你不会收到任何通知。
  • get 操作都是线程安全的， 并且 get 仅仅是从内存中 (mMap) 获取数据
  1. SharedPreferences不支持多进程，支持多线程。
  2. SharedPreference的文件备份机制
    SharedPreference的写入操作正式执行之前，首先会对文件进行备份，写入成功删除备份文件。若异常终止，则将备份文件重命名为源文件。
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// 尝试写入文件
private void writeToFile(...) {
  if (!backupFileExists) {
      !mFile.renameTo(mBackupFile);
  }
}
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// 写入成功，立即删除存在的备份文件
// Writing was successful, delete the backup file if there is one.
mBackupFile.delete();
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// 从磁盘初始化加载时执行
private void loadFromDisk() {
    synchronized (mLock) {
        if (mBackupFile.exists()) {
            mFile.delete();
            mBackupFile.renameTo(mFile);
        }
    }
  }