为什么我会得到“例外;必须被捕获或声明为被抛出“当我尝试编译我的Java代码时？

exception java

2022-08-31 22:34:49

考虑：

import java.awt.*;

import javax.swing.*;
import java.awt.event.*;
import javax.crypto.*;
import javax.crypto.spec.*;
import java.security.*;
import java.io.*;


public class EncryptURL extends JApplet implements ActionListener {

    Container content;
    JTextField userName = new JTextField();
    JTextField firstName = new JTextField();
    JTextField lastName = new JTextField();
    JTextField email = new JTextField();
    JTextField phone = new JTextField();
    JTextField heartbeatID = new JTextField();
    JTextField regionCode = new JTextField();
    JTextField retRegionCode = new JTextField();
    JTextField encryptedTextField = new JTextField();

    JPanel finishPanel = new JPanel();


    public void init() {

        //setTitle("Book - E Project");
        setSize(800, 600);
        content = getContentPane();
        content.setBackground(Color.yellow);
        content.setLayout(new BoxLayout(content, BoxLayout.Y_AXIS));

        JButton submit = new JButton("Submit");

        content.add(new JLabel("User Name"));
        content.add(userName);

        content.add(new JLabel("First Name"));
        content.add(firstName);

        content.add(new JLabel("Last Name"));
        content.add(lastName);

        content.add(new JLabel("Email"));
        content.add(email);

        content.add(new JLabel("Phone"));
        content.add(phone);

        content.add(new JLabel("HeartBeatID"));
        content.add(heartbeatID);

        content.add(new JLabel("Region Code"));
        content.add(regionCode);

        content.add(new JLabel("RetRegionCode"));
        content.add(retRegionCode);

        content.add(submit);

        submit.addActionListener(this);
    }


    public void actionPerformed(ActionEvent e) {

        if (e.getActionCommand() == "Submit"){

            String subUserName = userName.getText();
            String subFName = firstName.getText();
            String subLName = lastName.getText();
            String subEmail = email.getText();
            String subPhone = phone.getText();
            String subHeartbeatID = heartbeatID.getText();
            String subRegionCode = regionCode.getText();
            String subRetRegionCode = retRegionCode.getText();

            String concatURL =
                "user=" + subUserName + "&f=" + subFName +
                "&l=" + subLName + "&em=" + subEmail +
                "&p=" + subPhone + "&h=" + subHeartbeatID +
                "&re=" + subRegionCode + "&ret=" + subRetRegionCode;

            concatURL = padString(concatURL, ' ', 16);
            byte[] encrypted = encrypt(concatURL);
            String encryptedString = bytesToHex(encrypted);
            content.removeAll();
            content.add(new JLabel("Concatenated User Input -->" + concatURL));

            content.add(encryptedTextField);
            setContentPane(content);
        }
    }

    public static byte[] encrypt(String toEncrypt) throws Exception{
        try{
            String plaintext = toEncrypt;
            String key = "01234567890abcde";
            String iv = "fedcba9876543210";

            SecretKeySpec keyspec = new SecretKeySpec(key.getBytes(), "AES");
            IvParameterSpec ivspec = new IvParameterSpec(iv.getBytes());

            Cipher cipher = Cipher.getInstance("AES/CBC/NoPadding");
            cipher.init(Cipher.ENCRYPT_MODE, keyspec, ivspec);
            byte[] encrypted = cipher.doFinal(toEncrypt.getBytes());

            return encrypted;
        }
        catch(Exception e){
        }
    }


    public static byte[] decrypt(byte[] toDecrypt) throws Exception{
        String key = "01234567890abcde";
        String iv = "fedcba9876543210";

        SecretKeySpec keyspec = new SecretKeySpec(key.getBytes(), "AES");
        IvParameterSpec ivspec = new IvParameterSpec(iv.getBytes());

        Cipher cipher = Cipher.getInstance("AES/CBC/NoPadding");
        cipher.init(Cipher.DECRYPT_MODE, keyspec, ivspec);
        byte[] decrypted = cipher.doFinal(toDecrypt);

        return decrypted;
    }


    public static String bytesToHex(byte[] data) {
        if (data == null)
        {
            return null;
        }
        else
        {
            int len = data.length;
            String str = "";
            for (int i=0; i<len; i++)
            {
                if ((data[i]&0xFF) < 16)
                    str = str + "0" + java.lang.Integer.toHexString(data[i]&0xFF);
                else
                    str = str + java.lang.Integer.toHexString(data[i]&0xFF);
            }
            return str;
        }
    }


    public static String padString(String source, char paddingChar, int size)
    {
        int padLength = size-source.length() % size;
        for (int i = 0; i < padLength; i++) {
            source += paddingChar;
        }
        return source;
    }
}

我收到一个未报告的异常：

java.lang.Exception; must be caught or declared to be thrown
byte[] encrypted = encrypt(concatURL);

以及：

.java:109: missing return statement

如何解决这些问题？

答案 1

您的所有问题都源于此

byte[] encrypted = cipher.doFinal(toEncrypt.getBytes());
return encrypted;

哪些包含在 try，catch 块中，问题是，如果程序发现异常，则不会返回任何内容。像这样说（根据程序逻辑修改它）：

public static byte[] encrypt(String toEncrypt) throws Exception{
    try{
        String plaintext = toEncrypt;
        String key = "01234567890abcde";
        String iv = "fedcba9876543210";

        SecretKeySpec keyspec = new SecretKeySpec(key.getBytes(), "AES");
        IvParameterSpec ivspec = new IvParameterSpec(iv.getBytes());

        Cipher cipher = Cipher.getInstance("AES/CBC/NoPadding");
        cipher.init(Cipher.ENCRYPT_MODE,keyspec,ivspec);
        byte[] encrypted = cipher.doFinal(toEncrypt.getBytes());

        return encrypted;
    } catch(Exception e){
        return null;            // Always must return something
    }
}

对于第二个，您必须从加密方法调用中捕获异常，如下所示（也根据程序逻辑进行修改）：

public void actionPerformed(ActionEvent e)
  .
  .
  .
    try {
        byte[] encrypted = encrypt(concatURL);
        String encryptedString = bytesToHex(encrypted);
        content.removeAll();
        content.add(new JLabel("Concatenated User Input -->" + concatURL));

        content.add(encryptedTextField);
    setContentPane(content);
    } catch (Exception exc) {
        // TODO: handle exception
    }
}

您必须从中吸取的教训：

具有返回类型的方法必须始终返回该类型的对象，我的意思是在所有可能的情况下
必须始终处理所有选中的异常

答案 2

问题在于此方法：

  public static byte[] encrypt(String toEncrypt) throws Exception{

这是方法签名，它几乎说：

方法名称是什么：加密
它接收的参数：一个名为“加密”的字符串
其访问修饰符：公共静态
以及它是否在调用时可能会或不会引发异常。

在这种情况下，方法签名说，当调用此方法时，此方法“可能”可能会引发类型为“Exception”的异常。

    ....
    concatURL = padString(concatURL, ' ', 16);
    byte[] encrypted = encrypt(concatURL); <-- HERE!!!!!
    String encryptedString = bytesToHex(encrypted);
    content.removeAll();
    ......

所以编译器在说：要么你用一个 try/catch 构造来包围它，要么你声明方法（在哪里使用）来抛出“异常”它本身。

真正的问题是“加密”方法定义。任何方法都不应该返回“Exception”，因为它太通用了，并且可能隐藏一些其他类型的异常，更好的是有一个特定的异常。

试试这个：

public static byte[] encrypt(String toEncrypt) {
    try{
      String plaintext = toEncrypt;
      String key = "01234567890abcde";
      String iv = "fedcba9876543210";

      SecretKeySpec keyspec = new SecretKeySpec(key.getBytes(), "AES");
      IvParameterSpec ivspec = new IvParameterSpec(iv.getBytes());

      Cipher cipher = Cipher.getInstance("AES/CBC/NoPadding");
      cipher.init(Cipher.ENCRYPT_MODE,keyspec,ivspec);
      byte[] encrypted = cipher.doFinal(toEncrypt.getBytes());

      return encrypted;
    } catch ( NoSuchAlgorithmException nsae ) { 
        // What can you do if the algorithm doesn't exists??
        // this usually won't happen because you would test 
        // your code before shipping. 
        // So in this case is ok to transform to another kind 
        throw new IllegalStateException( nsae );
    } catch ( NoSuchPaddingException nspe ) { 
       // What can you do when there is no such padding ( whatever that means ) ??
       // I guess not much, in either case you won't be able to encrypt the given string
        throw new IllegalStateException( nsae );
    }
    // line 109 won't say it needs a return anymore.
  }

基本上，在这种特殊情况下，您应该确保加密包在系统中可用。

Java 需要加密包的扩展，因此，异常被声明为“已检查”异常。当它们不存在时，您可以处理。

在这个小程序中，如果加密包不可用，则无法执行任何操作，因此请在“开发”时进行检查。如果在程序运行时引发这些异常是因为您在“开发”中做错了什么，因此 RuntimeException 子类更合适。

最后一行不再需要 return 语句，在第一个版本中，你捕获异常并且不执行任何操作，这是错误的。

try { 
    // risky code ... 
} catch( Exception e ) { 
    // a bomb has just exploited
    // you should NOT ignore it 
} 

// The code continues here, but what should it do???

如果代码要失败，最好快速失败

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