java断点续传前后端交互，后端用AmazonS3实现示例代码

好的，下面是一个简单的示例代码：

前端代码：

var file = document.getElementById('file-input').files[0];
var fileName = file.name;
var fileSize = file.size;

// Ajax请求后端，获取已上传的文件大小
var uploadedSize = 0;
$.ajax({
    url: '/checkUploadedSize',
    type: 'GET',
    data: {fileName: fileName},
    success: function(response) {
        uploadedSize = response.uploadedSize;
        uploadFile(uploadedSize);
    },
    error: function() {
        console.log('Error occurred when checking uploaded size!');
    }
});

function uploadFile(uploadedSize) {
    var startByte = uploadedSize;
    var endByte = Math.min(fileSize, startByte + chunkSize);

    // 切分文件
    var chunk = file.slice(startByte, endByte);

    // 构造FormData对象，上传文件
    var formData = new FormData();
    formData.append('file', chunk, fileName);
    formData.append('startByte', startByte);
    formData.append('endByte', endByte);

    $.ajax({
        url: '/upload',
        type: 'POST',
        data: formData,
        processData: false,
        contentType: false,
        success: function(response) {
            if (endByte < fileSize) {
                uploadFile(endByte);
            } else {
                console.log('Upload complete!');
            }
        },
        error: function() {
            console.log('Error occurred when uploading file!');
        },
        xhr: function() {
            var xhr = new XMLHttpRequest();
            xhr.upload.addEventListener('progress', function(event) {
                var progress = Math.round((event.loaded + uploadedSize) / fileSize * 100);
                console.log('Upload progress: ' + progress + '%');
            });
            return xhr;
        }
    });
}

后端代码：

// 检查已上传的文件大小
app.get('/checkUploadedSize', function(req, res) {
    var fileName = req.query.fileName;

    // 根据文件名从Amazon S3获取已上传的文件大小
    var uploadedSize = getUploadedSize(fileName);

    res.json({uploadedSize: uploadedSize});
});

// 上传文件
app.post('/upload', function(req, res) {
    var file = req.files.file;
    var fileName = req.body.fileName;
    var startByte = parseInt(req.body.startByte);
    var endByte = parseInt(req.body.endByte);

    // 上传文件到Amazon S3
    uploadFileToS3(file, fileName, startByte, endByte);

    res.json({success: true});
});

其中，`getUploadedSize` 和 `uploadFileToS3` 分别是从 Amazon S3 获取已上传的文件大小和将文件上传到 Amazon S3 的函数。具体实现可以参考 Amazon S3 的官方文档。