遇到一個問題: 需要給所有的請求加簽名校驗以防刷接口;傳入請求url及body生成一個文本串作為一個header傳給服務端;已經有現成的簽名檢驗方法String doSignature(String url, byte[] body);當前網絡庫基於com.squareup.okhttp3:okhttp:3.14.2.
這很簡單了,當然是寫一個interceptor然后將request對象的url及body傳入就好.於是有:
public class SignInterceptor implements Interceptor {
@NonNull
@Override
public Response intercept(@NonNull Chain chain) throws IOException {
Request request = chain.request();
RequestBody body = request.body();
byte[] bodyBytes = null;
if (body != null) {
final Buffer buffer = new Buffer();
body.writeTo(buffer);
bodyBytes = buffer.readByteArray();
}
Request.Builder builder = request.newBuilder();
HttpUrl oldUrl = request.url();
final String url = oldUrl.toString();
final String signed = doSignature(url, bodyBytes));
if (!TextUtils.isEmpty(signed)) {
builder.addHeader(SIGN_KEY_NAME, signed);
}
return chain.proceed(builder.build());
}
}
okhttp的ReqeustBody是一個抽象類,內容輸出只有writeTo方法,將內容寫入到一個BufferedSink接口實現體里,然后再將數據轉成byte[]也就是內存數組.能達到目的的類只有Buffer,它實現了BufferedSink接口並能提供轉成內存數組的方法readByteArray. 這貌似沒啥問題呀,能造成OOM?
是的,要看請求類型,如果是一個上傳文件的接口呢?如果這個文件比較大呢?上傳接口有可能會用到public static RequestBody create(final @Nullable MediaType contentType, final File file)方法,如果是針對文件的實現體它的writeTo方法是sink.writeAll(source);而我們傳給簽名方法時用到的Buffer.readByteArray是將緩沖中的所有內容轉成了內存數組, 這意味着文件中的所有內容被轉成了內存數組, 就是在這個時機容易造成OOM! RequestBody.create源碼如下:
public static RequestBody create(final @Nullable MediaType contentType, final File file) {
if (file == null) throw new NullPointerException("file == null");
return new RequestBody() {
@Override public @Nullable MediaType contentType() {
return contentType;
}
@Override public long contentLength() {
return file.length();
}
@Override public void writeTo(BufferedSink sink) throws IOException {
try (Source source = Okio.source(file)) {
sink.writeAll(source);
}
}
};
}
可以看到實現體持有了文件,Content-Length返回了文件的大小, 內容全部轉給了Source對象。
這確實是以前非常容易忽略的一個點,很少有對請求體作額外處理的操作,而一旦這個操作變成一次性的大內存分配, 非常容易造成OOM. 所以要如何解決呢? 簽名方法又是如何處理的呢? 原來這個簽名方法在這里偷了個懶——它只讀取傳入body的前4K內容,然后只針對這部分內容進行了加密,至於傳入的這個內存數組本身多大並不考慮,完全把風險和麻煩丟給了外部(優秀的SDK!).
快速的方法當然是羅列白名單,針對上傳接口服務端不進行加簽驗證, 但這容易掛一漏萬,而且增加維護成本, 要簽名方法sdk的人另寫合適的接口等於要他們的命, 所以還是得從根本解決. 既然簽名方法只讀取前4K內容,我們便只將內容的前4K部分讀取再轉成方法所需的內存數組不就可了? 所以我們的目的是: 期望RequestBody能夠讀取一部分而不是全部的內容. 能否繼承RequestBody重寫它的writeTo? 可以,但不現實,不可能全部替代現有的RequestBody實現類, 同時ok框架也有可能創建私有的實現類. 所以只能針對writeTo的參數BufferedSink作文章, 先得了解BufferedSink又是如何被okhttp框架調用的.
BufferedSink相關的類包括Buffer, Source,都屬於okio框架,okhttp只是基於okio的一坨, okio沒有直接用java的io操作,而是另行寫了一套io操作,具體是數據緩沖的操作.接上面的描述, Source是怎么創建, 同時又是如何操作BufferedSink的? 在Okio.java中：
public static Source source(File file) throws FileNotFoundException {
if (file == null) throw new IllegalArgumentException("file == null");
return source(new FileInputStream(file));
}
public static Source source(InputStream in) {
return source(in, new Timeout());
}
private static Source source(final InputStream in, final Timeout timeout) {
return new Source() {
@Override public long read(Buffer sink, long byteCount) throws IOException {
try {
timeout.throwIfReached();
Segment tail = sink.writableSegment(1);
int maxToCopy = (int) Math.min(byteCount, Segment.SIZE - tail.limit);
int bytesRead = in.read(tail.data, tail.limit, maxToCopy);
if (bytesRead == -1) return -1;
tail.limit += bytesRead;
sink.size += bytesRead;
return bytesRead;
} catch (AssertionError e) {
if (isAndroidGetsocknameError(e)) throw new IOException(e);
throw e;
}
}
@Override public void close() throws IOException {
in.close();
}
@Override public Timeout timeout() {
return timeout;
}
};
}
Source把文件作為輸入流inputstream進行了各種讀操作, 但是它的read方法參數卻是個Buffer實例，它又是從哪來的，又怎么和BufferedSink關聯的？ 只好再繼續看BufferedSink.writeAll的實現體。
BufferedSink的實現類就是Buffer， 然后它的writeAll方法:
@Override public long writeAll(Source source) throws IOException {
if (source == null) throw new IllegalArgumentException("source == null");
long totalBytesRead = 0;
for (long readCount; (readCount = source.read(this, Segment.SIZE)) != -1; ) {
totalBytesRead += readCount;
}
return totalBytesRead;
}
原來是顯式的調用了Source.read(Buffer,long)方法，這樣就串起來了，那個Buffer參數原來就是自身。
基本可以確定只要實現BufferedSink接口類, 然后判斷讀入的內容超過指定大小就停止寫入就返回就可滿足目的, 可以名之FixedSizeSink.
然而麻煩的是BufferedSink的接口非常多, 將近30個方法， 不知道框架會在什么時機調用哪個方法，只能全部都實現! 其次是接口方法的參數有很多okio的類, 這些類的用法需要了解, 否則一旦用錯了效果適得其反. 於是對一個類的了解變成對多個類的了解, 沒辦法只能硬着頭皮寫.
第一個接口就有點蛋疼: Buffer buffer(); BufferedSink返回一個Buffer實例供外部調用, BufferedSink的實現體即是Buffer, 然后再返回一個Buffer?! 看了半天猜測BufferedSink是為了提供一個可寫入的緩沖對象, 但框架作者也懶的再搞接口解耦的那一套了(唉,大家都是怎么簡單怎么來). 於是FixedSizeSink至少需要持有一個Buffer對象, 它作實際的數據緩存，同時可以在需要Source.read(Buffer ,long)的地方作為參數傳過去.
同時可以看到RequestBody的一個實現類FormBody, 用這個Buffer對象直接寫入一些數據:
private long writeOrCountBytes(@Nullable BufferedSink sink, boolean countBytes) {
long byteCount = 0L;
Buffer buffer;
if (countBytes) {
buffer = new Buffer();
} else {
buffer = sink.buffer();
}
for (int i = 0, size = encodedNames.size(); i < size; i++) {
if (i > 0) buffer.writeByte('&');
buffer.writeUtf8(encodedNames.get(i));
buffer.writeByte('=');
buffer.writeUtf8(encodedValues.get(i));
}
if (countBytes) {
byteCount = buffer.size();
buffer.clear();
}
return byteCount;
}
有這樣的操作就有可能限制不了緩沖區大小變化！不過數據量應該相對小一些而且這種用法場景相對少，我們指定的大小應該能覆蓋的了這種情況。
接着還有一個接口BufferedSink write(ByteString byteString), 又得了解ByteString怎么使用, 真是心力交瘁啊...
@Override public Buffer write(ByteString byteString) {
byteString.write(this);
return this;
}
Buffer實現體里可以直接調用ByteString.write(Buffer)因為是包名訪問，自己實現的FixedSizeSink聲明在和同一包名package okio;也可以這樣使用，如果是其它包名只能先轉成byte[]了, ByteString應該不大不然也不能這么搞(沒有找到ByteString讀取一段數據的方法)：
@Override
public BufferedSink write(@NotNull ByteString byteString) throws IOException {
byte[] bytes = byteString.toByteArray();
this.write(bytes);
return this;
}
總之就是把這些對象轉成內存數組或者Buffer能夠接受的參數持有起來!
重點關心的writeAll反而相對好實現一點, 我們連續讀取指定長度的內容直到內容長度達到我們的閾值就行.
還有一個蛋疼的點是各種對象的read/write數據流方向:
Caller.read(Callee)/Caller.write(Callee), 有的是從Caller到Callee, 有的是相反，被一個小類整的有點頭疼……
最后上完整代碼, 如果發現什么潛在的問題也可以交流下~:
public class FixedSizeSink implements BufferedSink {
private static final int SEGMENT_SIZE = 4096;
private final Buffer mBuffer = new Buffer();
private final int mLimitSize;
private FixedSizeSink(int size) {
this.mLimitSize = size;
}
@Override
public Buffer buffer() {
return mBuffer;
}
@Override
public BufferedSink write(@NotNull ByteString byteString) throws IOException {
byte[] bytes = byteString.toByteArray();
this.write(bytes);
return this;
}
@Override
public BufferedSink write(@NotNull byte[] source) throws IOException {
this.write(source, 0, source.length);
return this;
}
@Override
public BufferedSink write(@NotNull byte[] source, int offset,
int byteCount) throws IOException {
long available = mLimitSize - mBuffer.size();
int count = Math.min(byteCount, (int) available);
android.util.Log.d(TAG, String.format("FixedSizeSink.offset=%d,"
"count=%d,limit=%d,size=%d",
offset, byteCount, mLimitSize, mBuffer.size()));
if (count > 0) {
mBuffer.write(source, offset, count);
}
return this;
}
@Override
public long writeAll(@NotNull Source source) throws IOException {
this.write(source, mLimitSize);
return mBuffer.size();
}
@Override
public BufferedSink write(@NotNull Source source, long byteCount) throws IOException {
final long count = Math.min(byteCount, mLimitSize - mBuffer.size());
final long BUFFER_SIZE = Math.min(count, SEGMENT_SIZE);
android.util.Log.d(TAG, String.format("FixedSizeSink.count=%d,limit=%d"
",size=%d,segment=%d",
byteCount, mLimitSize, mBuffer.size(), BUFFER_SIZE));
long totalBytesRead = 0;
long readCount;
while (totalBytesRead < count && (readCount = source.read(mBuffer, BUFFER_SIZE)) != -1) {
totalBytesRead = readCount;
}
return this;
}
@Override
public int write(ByteBuffer src) throws IOException {
final int available = mLimitSize - (int) mBuffer.size();
if (available < src.remaining()) {
byte[] bytes = new byte[available];
src.get(bytes);
this.write(bytes);
return bytes.length;
} else {
return mBuffer.write(src);
}
}
@Override
public void write(@NotNull Buffer source, long byteCount) throws IOException {
mBuffer.write(source, Math.min(byteCount, mLimitSize - mBuffer.size()));
}
@Override
public BufferedSink writeUtf8(@NotNull String string) throws IOException {
mBuffer.writeUtf8(string);
return this;
}
@Override
public BufferedSink writeUtf8(@NotNull String string, int beginIndex, int endIndex)
throws IOException {
mBuffer.writeUtf8(string, beginIndex, endIndex);
return this;
}
@Override
public BufferedSink writeUtf8CodePoint(int codePoint) throws IOException {
mBuffer.writeUtf8CodePoint(codePoint);
return this;
}
@Override
public BufferedSink writeString(@NotNull String string,
@NotNull Charset charset) throws IOException {
mBuffer.writeString(string, charset);
return this;
}
@Override
public BufferedSink writeString(@NotNull String string, int beginIndex, int endIndex,
@NotNull Charset charset) throws IOException {
mBuffer.writeString(string, beginIndex, endIndex, charset);
return this;
}
@Override
public BufferedSink writeByte(int b) throws IOException {
mBuffer.writeByte(b);
return this;
}
@Override
public BufferedSink writeShort(int s) throws IOException {
mBuffer.writeShort(s);
return this;
}
@Override
public BufferedSink writeShortLe(int s) throws IOException {
mBuffer.writeShortLe(s);
return this;
}
@Override
public BufferedSink writeInt(int i) throws IOException {
mBuffer.writeInt(i);
return this;
}
@Override
public BufferedSink writeIntLe(int i) throws IOException {
mBuffer.writeIntLe(i);
return this;
}
@Override
public BufferedSink writeLong(long v) throws IOException {
mBuffer.writeLong(v);
return this;
}
@Override
public BufferedSink writeLongLe(long v) throws IOException {
mBuffer.writeLongLe(v);
return this;
}
@Override
public BufferedSink writeDecimalLong(long v) throws IOException {
mBuffer.writeDecimalLong(v);
return this;
}
@Override
public BufferedSink writeHexadecimalUnsignedLong(long v) throws IOException {
mBuffer.writeHexadecimalUnsignedLong(v);
return this;
}
@Override
public void flush() throws IOException {
mBuffer.flush();
}
@Override
public BufferedSink emit() throws IOException {
mBuffer.emit();
return this;
}
@Override
public BufferedSink emitCompleteSegments() throws IOException {
mBuffer.emitCompleteSegments();
return this;
}
@Override
public OutputStream outputStream() {
return mBuffer.outputStream();
}
@Override
public boolean isOpen() {
return mBuffer.isOpen();
}
@Override
public Timeout timeout() {
return mBuffer.timeout();
}
@Override
public void close() throws IOException {
mBuffer.close();
}
}