Spring-MVC 文件上传优化

1. 配置 CommonsMultipartResolver 时把 maxInMemorySize 配置为合适的大小，让小文件可以缓存在内存中，对磁盘只需一次写操作；

2. 在 Controller 里调用 multipartFile.transferTo(file); 把文件保存到目标路径。该方法首先进行重命名，如果不成功则进行流拷贝，如果成功则可以省下一次读、写操作。对于 org.apache.commons.fileupload.disk.DiskFileItem 类，调用的方法是下面这个：

public void write(File file) throws Exception {
    if (isInMemory()) {
        FileOutputStream fout = null;
        try {
            fout = new FileOutputStream(file);
            fout.write(get());
        } finally {
            if (fout != null) {
                fout.close();
            }
        }
    } else {
        File outputFile = getStoreLocation();
        if (outputFile != null) {
            // Save the length of the file
            size = outputFile.length();
            /*
             * The uploaded file is being stored on disk
             * in a temporary location so move it to the
             * desired file.
             */
            if (!outputFile.renameTo(file)) {
                BufferedInputStream in = null;
                BufferedOutputStream out = null;
                try {
                    in = new BufferedInputStream(
                        new FileInputStream(outputFile));
                    out = new BufferedOutputStream(
                            new FileOutputStream(file));
                    IOUtils.copy(in, out);
                } finally {
                    if (in != null) {
                        try {
                            in.close();
                        } catch (IOException e) {
                            // ignore
                        }
                    }
                    if (out != null) {
                        try {
                            out.close();
                        } catch (IOException e) {
                            // ignore
                        }
                    }
                }
            }
        } else {
            /*
             * For whatever reason we cannot write the
             * file to disk.
             */
            throw new FileUploadException(
                "Cannot write uploaded file to disk!");
        }
    }
}

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