java.lang.RuntimeException:无法实例化活动组件信息[java]

您好，启动应用程序时出现错误：

03-09 16:51:36.53724639-24639/？E/AndroidRuntime:致命异常：主 进程：com.example.webserver，PID:24639 java.lang.RuntimeException:无法实例化活动组件信息{com.example.webserver/com.example.webserver.MainActivity}:java.lang.InstanceionException:java.lang.Class没有零参数构造函数 在android.app.ActivityThread.performLaunchActivity（ActivityThread.java:2515）上 位于android.app.ActivityThread.handleLaunchActivity（ActivityThread.java:2670） 在android.app.ActivityThread.access$1100（ActivityThread.java:178） 在android.app.ActivityThread$H.handleMessage（ActivityThread.java:1515）上 位于android.os.Handler.dispatchMessage（Handler.java:111） 位于android.os.Looper.loop（Looper.java:207） 位于android.app.ActivityThread.main（ActivityThread.java:5845） 位于java.lang.reflect.Method.invoke（本机方法） 在com.android.internal.os.ZygoteInit$MethodAndArgsCaller.run（ZygoteInit.java:907）上 位于com.android.internal.os.ZygoteInit.main（ZygoteInit.java:768） 原因：java.lang.InstanceException:java.lang.Class没有零参数构造函数 位于java.lang.Class.newInstance（本机方法） 位于android.app.Instrumentation.newActivity（Instrumentation.java:1072） 在android.app.ActivityThread.performLaunchActivity（ActivityThread.java:2505）上

我正在尝试在Android上制作一个web服务器。 代码如下：

    package com.example.webserver;

import androidx.appcompat.app.AppCompatActivity;

import java.io.BufferedOutputStream;
import java.io.BufferedReader;
import java.io.File;
import java.io.FileInputStream;
import java.io.FileNotFoundException;
import java.io.IOException;
import java.io.InputStreamReader;
import java.io.OutputStream;
import java.io.PrintWriter;
import java.net.ServerSocket;
import java.net.Socket;
import java.util.Date;
import java.util.StringTokenizer;

// The tutorial can be found just here on the SSaurel's Blog :
// https://www.ssaurel.com/blog/create-a-simple-http-web-server-in-java
// Each Client Connection will be managed in a dedicated Thread
public class MainActivity implements Runnable {

    static final File WEB_ROOT = new File(".");
    static final String DEFAULT_FILE = "index.html";
    static final String FILE_NOT_FOUND = "404.html";
    static final String METHOD_NOT_SUPPORTED = "not_supported.html";
    // port to listen connection
    static final int PORT = 8080;

    // verbose mode
    static final boolean verbose = true;

    // Client Connection via Socket Class
    private Socket connect;

    public MainActivity(Socket c) {
        connect = c;
    }

    public static void main(String[] args) {
        try {
            ServerSocket serverConnect = new ServerSocket(PORT);
            System.out.println("Server started.\nListening for connections on port : " + PORT + " ...\n");

            // we listen until user halts server execution
            while (true) {
                MainActivity myServer = new MainActivity(serverConnect.accept());

                if (verbose) {
                    System.out.println("Connecton opened. (" + new Date() + ")");
                }

                // create dedicated thread to manage the client connection
                Thread thread = new Thread(myServer);
                thread.start();
            }

        } catch (IOException e) {
            System.err.println("Server Connection error : " + e.getMessage());
        }
    }

    @Override
    public void run() {
        // we manage our particular client connection
        BufferedReader in = null; PrintWriter out = null; BufferedOutputStream dataOut = null;
        String fileRequested = null;

        try {
            // we read characters from the client via input stream on the socket
            in = new BufferedReader(new InputStreamReader(connect.getInputStream()));
            // we get character output stream to client (for headers)
            out = new PrintWriter(connect.getOutputStream());
            // get binary output stream to client (for requested data)
            dataOut = new BufferedOutputStream(connect.getOutputStream());

            // get first line of the request from the client
            String input = in.readLine();
            // we parse the request with a string tokenizer
            StringTokenizer parse = new StringTokenizer(input);
            String method = parse.nextToken().toUpperCase(); // we get the HTTP method of the client
            // we get file requested
            fileRequested = parse.nextToken().toLowerCase();

            // we support only GET and HEAD methods, we check
            if (!method.equals("GET")  &&  !method.equals("HEAD")) {
                if (verbose) {
                    System.out.println("501 Not Implemented : " + method + " method.");
                }

                // we return the not supported file to the client
                File file = new File(WEB_ROOT, METHOD_NOT_SUPPORTED);
                int fileLength = (int) file.length();
                String contentMimeType = "text/html";
                //read content to return to client
                byte[] fileData = readFileData(file, fileLength);

                // we send HTTP Headers with data to client
                out.println("HTTP/1.1 501 Not Implemented");
                out.println("Server: Java HTTP Server from SSaurel : 1.0");
                out.println("Date: " + new Date());
                out.println("Content-type: " + contentMimeType);
                out.println("Content-length: " + fileLength);
                out.println(); // blank line between headers and content, very important !
                out.flush(); // flush character output stream buffer
                // file
                dataOut.write(fileData, 0, fileLength);
                dataOut.flush();

            } else {
                // GET or HEAD method
                if (fileRequested.endsWith("/")) {
                    fileRequested += DEFAULT_FILE;
                }

                File file = new File(WEB_ROOT, fileRequested);
                int fileLength = (int) file.length();
                String content = getContentType(fileRequested);

                if (method.equals("GET")) { // GET method so we return content
                    byte[] fileData = readFileData(file, fileLength);

                    // send HTTP Headers
                    out.println("HTTP/1.1 200 OK");
                    out.println("Server: Java HTTP Server from SSaurel : 1.0");
                    out.println("Date: " + new Date());
                    out.println("Content-type: " + content);
                    out.println("Content-length: " + fileLength);
                    out.println(); // blank line between headers and content, very important !
                    out.flush(); // flush character output stream buffer

                    dataOut.write(fileData, 0, fileLength);
                    dataOut.flush();
                }

                if (verbose) {
                    System.out.println("File " + fileRequested + " of type " + content + " returned");
                }

            }

        } catch (FileNotFoundException fnfe) {
            try {
                fileNotFound(out, dataOut, fileRequested);
            } catch (IOException ioe) {
                System.err.println("Error with file not found exception : " + ioe.getMessage());
            }

        } catch (IOException ioe) {
            System.err.println("Server error : " + ioe);
        } finally {
            try {
                in.close();
                out.close();
                dataOut.close();
                connect.close(); // we close socket connection
            } catch (Exception e) {
                System.err.println("Error closing stream : " + e.getMessage());
            }

            if (verbose) {
                System.out.println("Connection closed.\n");
            }
        }


    }

    private byte[] readFileData(File file, int fileLength) throws IOException {
        FileInputStream fileIn = null;
        byte[] fileData = new byte[fileLength];

        try {
            fileIn = new FileInputStream(file);
            fileIn.read(fileData);
        } finally {
            if (fileIn != null)
                fileIn.close();
        }

        return fileData;
    }

    // return supported MIME Types
    private String getContentType(String fileRequested) {
        if (fileRequested.endsWith(".htm")  ||  fileRequested.endsWith(".html"))
            return "text/html";
        else
            return "text/plain";
    }

    private void fileNotFound(PrintWriter out, OutputStream dataOut, String fileRequested) throws IOException {
        File file = new File(WEB_ROOT, FILE_NOT_FOUND);
        int fileLength = (int) file.length();
        String content = "text/html";
        byte[] fileData = readFileData(file, fileLength);

        out.println("HTTP/1.1 404 File Not Found");
        out.println("Server: Java HTTP Server from SSaurel : 1.0");
        out.println("Date: " + new Date());
        out.println("Content-type: " + content);
        out.println("Content-length: " + fileLength);
        out.println(); // blank line between headers and content, very important !
        out.flush(); // flush character output stream buffer

        dataOut.write(fileData, 0, fileLength);
        dataOut.flush();

        if (verbose) {
            System.out.println("File " + fileRequested + " not found");
        }
    }

}

---

如果这是您的

main活动

，并且不存在其他活动，则启动时出错。您的

AndroidManifest

文件可能指向这个

MainActivity.java

类，但由于它扩展了

Runnable

而不是

AppCompat

，所以它不知道该做什么。导致构造函数错误

为了解决这个问题，我将创建另一个扩展AppCompat的

Main.java

类，从那里运行活动，然后从

onCreate

方法启动此

Main活动Runnable