SharePreference为什么不适合存储大容量数据

操作简单，以键值对的方式存储在本地。

getSharedPreference在子线程进行读取操作，为啥还会阻塞线程呢? 因为get操作要等所有键值对都读取完毕之后，才能进行操作。

看源码：

val sp = getSharedPreferences("zhen", Context.MODE_PRIVATE)
val editor = sp.edit()
editor.putString("key", "value")
editor.commit()

val value = sp.getString("key", "null")

SharedPreferenceImpl

SharedPreferencesImpl(File file, int mode) {
    mFile = file;
    mBackupFile = makeBackupFile(file);
    mMode = mode;
    mLoaded = false;
    mMap = null;
    startLoadFromDisk();
}

开启子线程进行文件读取

private void startLoadFromDisk() {
    synchronized (mLock) {
        mLoaded = false;
    }
    new Thread("SharedPreferencesImpl-load") {
        public void run() {
            loadFromDisk();
        }
    }.start();
}

private void loadFromDisk() {
    synchronized (mLock) {
        if (mLoaded) {
            return;
        }
        if (mBackupFile.exists()) {
            mFile.delete();
            mBackupFile.renameTo(mFile);
        }
    }

    // Debugging
    if (mFile.exists() && !mFile.canRead()) {
        Log.w(TAG, "Attempt to read preferences file " + mFile + " without permission");
    }

    Map map = null;
    StructStat stat = null;
    try {
        stat = Os.stat(mFile.getPath());
        if (mFile.canRead()) {
            BufferedInputStream str = null;
            try {
                str = new BufferedInputStream(
                        new FileInputStream(mFile), 16*1024);
                map = XmlUtils.readMapXml(str);
            } catch (Exception e) {
                Log.w(TAG, "Cannot read " + mFile.getAbsolutePath(), e);
            } finally {
                IoUtils.closeQuietly(str);
            }
        }
    } catch (ErrnoException e) {
        /* ignore */
    }

    synchronized (mLock) {
        mLoaded = true;
        if (map != null) {
            mMap = map;
            mStatTimestamp = stat.st_mtim;
            mStatSize = stat.st_size;
        } else {
            mMap = new HashMap<>();
        }
        mLock.notifyAll();
    }
}

那为啥在子线程操作，还会阻塞UI呢？

@Nullable
public String getString(String key, @Nullable String defValue) {
    synchronized (mLock) {
        awaitLoadedLocked();
        String v = (String)mMap.get(key);
        return v != null ? v : defValue;
    }
}

//会等待所有的都读取完毕之后，才进行get操作

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) {
        }
    }
}

//put操作会使用SharedPreference的内部类EditorImpl, 统一commit。

public Editor putString(String key, @Nullable String value) {
    synchronized (mLock) {
        mModified.put(key, value);
        return this;
    }
}

public boolean commit() {
    long startTime = 0;

    if (DEBUG) {
        startTime = System.currentTimeMillis();
    }

    MemoryCommitResult mcr = commitToMemory();

    SharedPreferencesImpl.this.enqueueDiskWrite(
        mcr, null /* sync write on this thread okay */);
    try {
        mcr.writtenToDiskLatch.await();
    } catch (InterruptedException e) {
        return false;
    } finally {
        if (DEBUG) {
            Log.d(TAG, mFile.getName() + ":" + mcr.memoryStateGeneration
                    + " committed after " + (System.currentTimeMillis() - startTime)
                    + " ms");
        }
    }
    notifyListeners(mcr);
    return mcr.writeToDiskResult;
}

在xml文件全部内加载到内存中之前，读取操作是阻塞的，在xml文件全部内加载到内存中之后，是直接读取内存中的数据