对底层的保存机制不大清楚。不过发现了一个“BUG”，原位保存特定文件的的时候，保存后出现乱码，但若测试“另存为”其他文件，一切正常。这个“BUG”说明，PDFium在保存文件的时候，可能发生边读边写的情况，一旦写指针超过读指针，就会发生未知异常。所以，需要保证“写”落后于“读”，也就是使用Java中BufferedOutputStream的思想。
但是Jni怎么调用BufferedOutputStream？在jni层，写入文件是直接通过write函数完成的。所以只需为write函数包上一层Buffered方法即可，无需Jni call来call去。
参照 BufferedOutputStream.java，很简单地实现如下：
BufferedBlockWriter.h
#ifndef _BBW_
#define _BBW_
#include <fpdf_save.h>
// Start Save PDF
struct PdfToFdWriter : FPDF_FILEWRITE {
int dstFd;
};
void flushBuffer(int fd);
bool writeAllBytes(const int fd, const void *buffer, const size_t byteCount);
int writeBlock(FPDF_FILEWRITE* owner, const void* buffer, unsigned long size);
int writeBlockBuffered(FPDF_FILEWRITE* owner, const void* buffer, unsigned long size);
void startBufferedWriting(size_t buffer_size);
#endif
BufferedBlockWriter.cpp
#include "util.hpp"
extern "C" {
#include <unistd.h>
#include <sys/mman.h>
#include <sys/stat.h>
#include <string.h>
#include <stdio.h>
#include "BufferedBlockWriter.h"
}
#include <android/native_window.h>
#include <android/native_window_jni.h>
#include <android/bitmap.h>
#include <utils/Mutex.h>
using namespace android;
#include <fpdfview.h>
#include <fpdf_doc.h>
#include <fpdf_text.h>
#include <fpdf_annot.h>
#include <fpdf_save.h>
#include <string>
#include <vector>
#include "BufferedBlockWriter.h"
bool writeAllBytes(const int fd, const void *buffer, const size_t byteCount) {
char *writeBuffer = static_cast<char *>(const_cast<void *>(buffer));
size_t remainingBytes = byteCount;
LOGE("fatal writeAllBytes: %d %d %d", buffer, byteCount, remainingBytes);
while (remainingBytes > 0) {
ssize_t writtenByteCount = write(fd, writeBuffer, remainingBytes);
if (writtenByteCount == -1) {
if (errno == EINTR) {
continue;
}
LOGE("fatal Error writing to buffer: %d", errno);
return true;
}
remainingBytes -= writtenByteCount;
writeBuffer += writtenByteCount;
}
return true;
}
int writeBlock(FPDF_FILEWRITE* owner, const void* buffer, unsigned long size) {
LOGE("fatal writeBlock: %d %d %d", buffer, size, owner);
const PdfToFdWriter* writer = reinterpret_cast<PdfToFdWriter*>(owner);
const bool success = writeAllBytes(writer->dstFd, buffer, size);
if (!success) {
LOGE("fatal Cannot write to file descriptor. Error:%d", errno);
return 0;
}
return 1;
}
// 以下实现 buffered 方法
static char* buf = new char[1024*1024*4];
static size_t buf_length = 1024*1024*4;
static size_t count;
int error = 0;
void flushBuffer(int fd) {
LOGE("fatal flushBuffer count=%d buf_length=%d", count, buf_length);
if (count > 0) {
if(!writeAllBytes(fd, buf, count)) {
error = 1;
}
count = 0;
}
}
bool writeAllBytesBuffered(const int fd, const void *buffer, const size_t len) {
if (len >= buf_length) {
/* If the request length exceeds the size of the output buffer,
flush the output buffer and then write the data directly.
In this way buffered streams will cascade harmlessly. */
flushBuffer(fd);
return writeAllBytes(fd, buffer, len);
}
if (len > buf_length - count) {
flushBuffer(fd);
}
if(error) {
return false;
}
memcpy(buf+count, buffer, len);
count += len;
return true;
}
int writeBlockBuffered(FPDF_FILEWRITE* owner, const void* buffer, unsigned long size) {
LOGE("fatal writeBlock: %d %d %d", buffer, size, owner);
const PdfToFdWriter* writer = reinterpret_cast<PdfToFdWriter*>(owner);
const bool success = writeAllBytesBuffered(writer->dstFd, buffer, size);
if (!success) {
LOGE("fatal Cannot write to file descriptor. Error:%d", errno);
return 0;
}
//flushBuffer(writer->dstFd);
return 1;
}
void startBufferedWriting(size_t buffer_size) {
count = 0;
}
使用（PDFium实现原位保存）：
JNI_FUNC(void, PdfiumCore, nativeSaveAsCopy)(JNI_ARGS, jlong docPtr, jint fd, jboolean incremental) {
DocumentFile* docFile = (DocumentFile*)docPtr;
PdfToFdWriter writer;
writer.dstFd = fd;
//writer.dstFd = docFile->fileFd;
if(false) {
writer.WriteBlock = &writeBlock;
} else {
writer.WriteBlock = &writeBlockBuffered;
startBufferedWriting(0);
}
FPDF_BOOL success = FPDF_SaveAsCopy(docFile->pdfDocument, &writer, incremental?FPDF_INCREMENTAL:FPDF_NO_INCREMENTAL); // FPDF_INCREMENTAL FPDF_NO_INCREMENTAL
if (!success) {
jniThrowExceptionFmt(env, "java/io/IOException", "cannot write to fd. Error: %d", errno);
} else {
flushBuffer(fd);
}
}
附：出现“BUG”的文件
https://comserv.cs.ut.ee/ati_thesis/datasheet.php?id=61865&year=2018&language=en
BUG 图示
事实上不能完全解决问题。打印发现，很多情况（比如Gpu pro 1）下PDFium保存末端文件块的时候需要读取偏移为17、大小为512字节、已经被覆盖了的文件块。 另一些情况（比如Gpu pro 4）则会读取偏移更大的已覆写文件块，毫无规律。所以，最终还是通过将PDF文件整个加载进内存解决问题。不过，这个 BufferedWriter 写入方法还是可以保留的。
网友评论