通过内容提供程序传递二进制 blob

Question

我有一个针对我的一组 Android 应用程序定制的内容提供程序，它需要公开的内容之一是一个小型（20-30 KiB）字节数组。这些 blob 的 URI 如下所示：

content://my.authority/blob/#

其中 # 是行号；生成的游标具有标准的 _id 列和数据列。我在提供程序的 query() 方法中使用 MatrixCursor：

byte[] byteData = getMyByteData();
MatrixCursor mc = new MatrixCursor(COLUMNS);
mc.addRow(new Object[] { id, byteData });

稍后，在使用数据的应用程序中，我这样做：

Cursor c = managedQuery(uri, null, null, null, null);
c.moveToFirst();
byte[] data = c.getBlob(c.getColumnIndexOrThrow("data"));

但是，数据不包含我的内容原始字节数组；相反，它包含类似 [B@435cc518 的内容，它看起来更像是数组的地址而不是内容。我尝试将字节数组包装在 java.sql.Blob 的实现中，认为它可能正在寻找该字节数组，因为内容提供程序子系统被编写为易于与 SQLite 一起使用，但事实并非如此。没有帮助。

有人让它发挥作用吗？如果数据位于文件系统中，我可以使用 ContentProvider 中的方法向客户端提供编组的 InputStream，但我尝试发送的数据back 作为内容提供商的 APK 中的资源存在。

Answer 1

从 Ice Cream Sandwich 源获取 MatrixCursor。它正确实现了getBlob。

下面是修改后的源代码，如果你想使用它。您可以在 Android 1.6+ 项目中使用它。

/*
 * Copyright (C) 2007 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 yuku.androidsdk.android.database.icsmatrixcursor;

import android.database.*;

import java.util.*;

/**
 * A mutable cursor implementation backed by an array of {@code Object}s. Use
 * {@link #newRow()} to add rows. Automatically expands internal capacity
 * as needed.
 */
public class MatrixCursor extends AbstractCursor {

    private final String[] columnNames;
    private Object[] data;
    private int rowCount = 0;
    private final int columnCount;

    /**
     * Constructs a new cursor with the given initial capacity.
     *
     * @param columnNames names of the columns, the ordering of which
     *  determines column ordering elsewhere in this cursor
     * @param initialCapacity in rows
     */
    public MatrixCursor(String[] columnNames, int initialCapacity) {
        this.columnNames = columnNames;
        this.columnCount = columnNames.length;

        if (initialCapacity < 1) {
            initialCapacity = 1;
        }

        this.data = new Object[columnCount * initialCapacity];
    }

    /**
     * Constructs a new cursor.
     *
     * @param columnNames names of the columns, the ordering of which
     *  determines column ordering elsewhere in this cursor
     */
    public MatrixCursor(String[] columnNames) {
        this(columnNames, 16);
    }

    /**
     * Gets value at the given column for the current row.
     */
    private Object get(int column) {
        if (column < 0 || column >= columnCount) {
            throw new CursorIndexOutOfBoundsException("Requested column: "
                    + column + ", # of columns: " +  columnCount);
        }
        if (mPos < 0) {
            throw new CursorIndexOutOfBoundsException("Before first row.");
        }
        if (mPos >= rowCount) {
            throw new CursorIndexOutOfBoundsException("After last row.");
        }
        return data[mPos * columnCount + column];
    }

    /**
     * Adds a new row to the end and returns a builder for that row. Not safe
     * for concurrent use.
     *
     * @return builder which can be used to set the column values for the new
     *  row
     */
    public RowBuilder newRow() {
        rowCount++;
        int endIndex = rowCount * columnCount;
        ensureCapacity(endIndex);
        int start = endIndex - columnCount;
        return new RowBuilder(start, endIndex);
    }

    /**
     * Adds a new row to the end with the given column values. Not safe
     * for concurrent use.
     *
     * @throws IllegalArgumentException if {@code columnValues.length !=
     *  columnNames.length}
     * @param columnValues in the same order as the the column names specified
     *  at cursor construction time
     */
    public void addRow(Object[] columnValues) {
        if (columnValues.length != columnCount) {
            throw new IllegalArgumentException("columnNames.length = "
                    + columnCount + ", columnValues.length = "
                    + columnValues.length);
        }

        int start = rowCount++ * columnCount;
        ensureCapacity(start + columnCount);
        System.arraycopy(columnValues, 0, data, start, columnCount);
    }

    /**
     * Adds a new row to the end with the given column values. Not safe
     * for concurrent use.
     *
     * @throws IllegalArgumentException if {@code columnValues.size() !=
     *  columnNames.length}
     * @param columnValues in the same order as the the column names specified
     *  at cursor construction time
     */
    public void addRow(Iterable<?> columnValues) {
        int start = rowCount * columnCount;
        int end = start + columnCount;
        ensureCapacity(end);

        if (columnValues instanceof ArrayList<?>) {
            addRow((ArrayList<?>) columnValues, start);
            return;
        }

        int current = start;
        Object[] localData = data;
        for (Object columnValue : columnValues) {
            if (current == end) {
                // TODO: null out row?
                throw new IllegalArgumentException(
                        "columnValues.size() > columnNames.length");
            }
            localData[current++] = columnValue;
        }

        if (current != end) {
            // TODO: null out row?
            throw new IllegalArgumentException(
                    "columnValues.size() < columnNames.length");
        }

        // Increase row count here in case we encounter an exception.
        rowCount++;
    }

    /** Optimization for {@link ArrayList}. */
    private void addRow(ArrayList<?> columnValues, int start) {
        int size = columnValues.size();
        if (size != columnCount) {
            throw new IllegalArgumentException("columnNames.length = "
                    + columnCount + ", columnValues.size() = " + size);
        }

        rowCount++;
        Object[] localData = data;
        for (int i = 0; i < size; i++) {
            localData[start + i] = columnValues.get(i);
        }
    }

    /** Ensures that this cursor has enough capacity. */
    private void ensureCapacity(int size) {
        if (size > data.length) {
            Object[] oldData = this.data;
            int newSize = data.length * 2;
            if (newSize < size) {
                newSize = size;
            }
            this.data = new Object[newSize];
            System.arraycopy(oldData, 0, this.data, 0, oldData.length);
        }
    }

    /**
     * Builds a row, starting from the left-most column and adding one column
     * value at a time. Follows the same ordering as the column names specified
     * at cursor construction time.
     */
    public class RowBuilder {

        private int index;
        private final int endIndex;

        RowBuilder(int index, int endIndex) {
            this.index = index;
            this.endIndex = endIndex;
        }

        /**
         * Sets the next column value in this row.
         *
         * @throws CursorIndexOutOfBoundsException if you try to add too many
         *  values
         * @return this builder to support chaining
         */
        public RowBuilder add(Object columnValue) {
            if (index == endIndex) {
                throw new CursorIndexOutOfBoundsException(
                        "No more columns left.");
            }

            data[index++] = columnValue;
            return this;
        }
    }

    // AbstractCursor implementation.

    @Override
    public int getCount() {
        return rowCount;
    }

    @Override
    public String[] getColumnNames() {
        return columnNames;
    }

    @Override
    public String getString(int column) {
        Object value = get(column);
        if (value == null) return null;
        return value.toString();
    }

    @Override
    public short getShort(int column) {
        Object value = get(column);
        if (value == null) return 0;
        if (value instanceof Number) return ((Number) value).shortValue();
        return Short.parseShort(value.toString());
    }

    @Override
    public int getInt(int column) {
        Object value = get(column);
        if (value == null) return 0;
        if (value instanceof Number) return ((Number) value).intValue();
        return Integer.parseInt(value.toString());
    }

    @Override
    public long getLong(int column) {
        Object value = get(column);
        if (value == null) return 0;
        if (value instanceof Number) return ((Number) value).longValue();
        return Long.parseLong(value.toString());
    }

    @Override
    public float getFloat(int column) {
        Object value = get(column);
        if (value == null) return 0.0f;
        if (value instanceof Number) return ((Number) value).floatValue();
        return Float.parseFloat(value.toString());
    }

    @Override
    public double getDouble(int column) {
        Object value = get(column);
        if (value == null) return 0.0d;
        if (value instanceof Number) return ((Number) value).doubleValue();
        return Double.parseDouble(value.toString());
    }

    @Override
    public byte[] getBlob(int column) {
        Object value = get(column);
        return (byte[]) value;
    }

    @Override
    public boolean isNull(int column) {
        return get(column) == null;
    }
}

Answer 2

您将无法使用 MatrixCursor 发送字节数组。这是因为它依赖于 AbstractCursor#fillWindow 方法，该方法使用 Object#toString 填充 CursorWindow。因此，正在发生的事情是调用字节数组 toString 方法，并且它存储该方法，而不是您想要的字节数组的内容。我能看到的解决这个问题的唯一方法是实现您自己的游标，它将为字节数组适当地填充 CursorWindow 。

Answer 3

您需要在 AbstractCursor 实现中重写 fillWindow。这是适用于 JUST BLOBS 的内容提供商的一个

        public void fillWindow(int position, CursorWindow window) {
            if (position < 0 || position >= getCount()) {
                return;
            }
            window.acquireReference();
            try {
                int oldpos = mPos;
                mPos = position - 1;
                window.clear();
                window.setStartPosition(position);
                int columnNum = getColumnCount();
                window.setNumColumns(columnNum);
                while (moveToNext() && window.allocRow()) {            
                    for (int i = 0; i < columnNum; i++) {
                        byte [] field = getBlob(i);
                        if (field != null) {
                            if (!window.putBlob(field, mPos, i)) {
                                window.freeLastRow();
                                break;
                            }
                        } else {
                            if (!window.putNull(mPos, i)) {
                                window.freeLastRow();
                                break;
                            }
                        }
                    }
                }

                mPos = oldpos;
            } catch (IllegalStateException e){
                // simply ignore it
            } finally {
                window.releaseReference();
            }
        }