java - 从被调用方传递的参数但在方法定义中为 null

Question

我基本上是在尝试创建一个解压缩功能。我在下面的 block 中调用了带参数的函数:

UnzipUtility unzipUtility = new UnzipUtility();
    try {
        unzipUtility.unzip(localFilePath, parentPath);
    } catch (IOException e) {
        e.printStackTrace();
    }

该方法定义在类UnzipUtility中，代码如下:

    public void unzip(String zipFilePath, String destDirectory) throws IOException {
    File destDir = new File(destDirectory);
    if (!destDir.exists()) {
        destDir.mkdir();
    }
    ZipInputStream zipIn = new ZipInputStream(new FileInputStream(zipFilePath));
    ZipEntry entry = zipIn.getNextEntry();
    // iterates over entries in the zip file
    while (entry != null) {
        String filePath = destDirectory + File.separator + entry.getName();
        if (!entry.isDirectory()) {
            // if the entry is a file, extracts it
            extractFile(zipIn, filePath);
        } else {
            // if the entry is a directory, make the directory
            File dir = new File(filePath);
            dir.mkdir();
        }
        zipIn.closeEntry();
        entry = zipIn.getNextEntry();
    }
    zipIn.close();
}

但是在运行时，虽然在主类中正确传递了参数，但在解压缩方法中这些值显示为 null。

请帮忙解决这个问题

主要类如下:

package com.example.sftpconnection;
import android.annotation.SuppressLint;
import android.os.AsyncTask;
import android.support.v7.app.AppCompatActivity;
import android.os.Bundle;
import android.util.Log;
import com.jcraft.jsch.Channel;
import com.jcraft.jsch.ChannelSftp;
import com.jcraft.jsch.JSch;
import com.jcraft.jsch.Session;
import com.jcraft.jsch.JSchException;
import com.jcraft.jsch.SftpException;


//import java.io.BufferedOutputStream;
import java.io.File;
//import java.io.FileOutputStream;
import java.io.IOException;
//import java.io.OutputStream;
import java.util.List;
import com.example.sftpconnection.UnzipUtility;

public class SFTPActivity extends AppCompatActivity {

    private String fileName = "1234.zip";
    private String localFilePath;
    private String parentPath;
    @SuppressLint("StaticFieldLeak")
    @Override
    protected void onCreate(Bundle savedInstanceState) {
        super.onCreate(savedInstanceState);
        setContentView(R.layout.activity_sftp);


        new AsyncTask<Void, Void, List<String>>() {
            @Override
            protected List<String> doInBackground(Void... params) {
                try {
                    Downloader(fileName);
                } catch (Exception e) {
                    e.printStackTrace();
                }
                return null;
            }

        }.execute();

        UnzipUtility unzipUtility = new UnzipUtility();
        try {
            unzipUtility.unzip(localFilePath, parentPath);
        } catch (IOException e) {
            e.printStackTrace();
        }
    }

    public void Downloader(String fileName) {

        String user = "1234";
        String pass = "1234";
        String host = "1234";
        int portNum = 22;

        JSch jsch = new JSch();
        Session session;

        try {

            session = jsch.getSession(user,host,portNum);
            session.setConfig("StrictHostKeyChecking", "no");
            session.setPassword(pass);
            session.connect();

            Channel channel = session.openChannel("sftp");
            channel.connect();
            ChannelSftp sftpChannel = (ChannelSftp) channel;


            File localFile = File.createTempFile("1234",".zip");
            sftpChannel.get(fileName,localFile.getAbsolutePath());
            //sftpChannel.get(fileName);

            Log.d(fileName, " has been downloaded");

            sftpChannel.exit();
            session.disconnect();
            localFilePath = localFile.getAbsolutePath();
            parentPath = localFile.getParent();
        } catch (JSchException | IOException | SftpException e) {
            e.printStackTrace();
        }
    }


}

出于安全原因，我编辑了数据字段。

Answer 1

可以剪掉下面几行:

UnzipUtility unzipUtility = new UnzipUtility();
try {
    unzipUtility.unzip(localFilePath, parentPath);
} catch (IOException e) {
    e.printStackTrace();
}

在类 SFTPActivity 中粘贴以下行:

Downloader(fileName); 

方法 doInBackground。实际上，doInBackground() 方法在与 onCreate 方法运行的线程不同的线程中运行。您尝试在方法 Downloader(fileName) 完成其工作之前使用这些变量。这就是为什么您在以下变量中看到空值的原因:localFilePath 和 parentPath