Question

即使在try块中返回，它也表示缺少return语句。我不明白。我正在尝试制作一个android应用程序，并且还使用android keystore存储该应用程序的登录凭据。另外，如果有人可以给我提供简单的keystore实现示例，那就太好了。我发现了2个示例，它们不是很容易理解（缺少代码），并且也很难实现我的情况。

private SecretKey createKey() {
    try {
        KeyGenerator keyGenerator = KeyGenerator.getInstance(KeyProperties.KEY_ALGORITHM_AES, "AndroidKeyStore");
        if (Build.VERSION.SDK_INT >= Build.VERSION_CODES.M) {
            keyGenerator.init(new KeyGenParameterSpec.Builder("Key", KeyProperties.PURPOSE_ENCRYPT | KeyProperties.PURPOSE_DECRYPT)
                    .setBlockModes(KeyProperties.BLOCK_MODE_CBC)
                    .setUserAuthenticationRequired(true)            //burayı kaldırırsan screen locka gerek kalmaz
                    .setUserAuthenticationValidityDurationSeconds(5)
                    .setEncryptionPaddings(KeyProperties.ENCRYPTION_PADDING_PKCS7)
                    .build());
            return keyGenerator.generateKey();
        }
    } catch (NoSuchAlgorithmException | NoSuchProviderException | InvalidAlgorithmParameterException e) {
        throw new RuntimeException("Failed to create a symmetric key", e);
    }

}

Answer 1

问题是，如果您的Build.VERSION.SDK_INT >= Build.VERSION_CODES.M条件没有得到满足，您就没有回报。

private SecretKey createKey() {
    try {
        KeyGenerator keyGenerator = KeyGenerator.getInstance(KeyProperties.KEY_ALGORITHM_AES, "AndroidKeyStore");
        if (Build.VERSION.SDK_INT >= Build.VERSION_CODES.M) {
            // ...
            return keyGenerator.generateKey();
        }
        // add a return here if we're not on >= Android M.
    } catch (NoSuchAlgorithmException | NoSuchProviderException | InvalidAlgorithmParameterException e) {
        throw new RuntimeException("Failed to create a symmetric key", e);
    }
    // you could also have a return here.
}

Answer 2

如果if语句不符合要求，则需要运行return语句。现在，只有在SDK大于Build.VERSION_CODES.M时，您才返回某些内容。

您可以在整个try and catch之后放置一个return语句，因为如果您在try中返回，它将离开该方法。这意味着只有在构建SDK不符合要求时，才会运行其他返回。

private SecretKey createKey() {
try {
    KeyGenerator keyGenerator = KeyGenerator.getInstance(KeyProperties.KEY_ALGORITHM_AES, "AndroidKeyStore");
    if (Build.VERSION.SDK_INT >= Build.VERSION_CODES.M) {
        keyGenerator.init(new KeyGenParameterSpec.Builder("Key", KeyProperties.PURPOSE_ENCRYPT | KeyProperties.PURPOSE_DECRYPT)
                .setBlockModes(KeyProperties.BLOCK_MODE_CBC)
                .setUserAuthenticationRequired(true)            //burayı kaldırırsan screen locka gerek kalmaz
                .setUserAuthenticationValidityDurationSeconds(5)
                .setEncryptionPaddings(KeyProperties.ENCRYPTION_PADDING_PKCS7)
                .build());
        return keyGenerator.generateKey();
    }
} catch (NoSuchAlgorithmException | NoSuchProviderException | InvalidAlgorithmParameterException e) {
    throw new RuntimeException("Failed to create a symmetric key", e);
}

// Return here...

}

Answer 3

在最后一个花括号之前添加return语句

catch {throw ..}实现后缺少返回语句

3 个答案: