I recently had a situation where I had a JAR that had dependencies on another JAR, but wouldn’t always be packaged with the other JAR.\u00a0 I’m not completely happy with what I ended up doing, but since I succeeded in doing what I wasn’t certain was possible, I decided to document it.\u00a0 Basically (as the title suggests), I tweaked my code so that at runtime it will download the JAR and add it to the classpath.<\/p>\n
Normally I’d use something like the Apache Commons IO<\/a> library to help with downloading the JAR, but since that’s one of the pieces in the JAR to be downloaded, I’m in a catch-22 situation.\u00a0 Instead, I used vanilla Java for the implementation.\u00a0 I ran into some minor complications because the server hosting the JAR did not have a signed certificate, so I had to force Java to ignore certificate errors.\u00a0 Fortunately, no authentication was required–otherwise things would have been a bit more complicated.\u00a0 Here is my class:<\/p>\n This was my first foray into dynamically changing the classpath at runtime.\u00a0 I found many examples of how to load a specific class (when you know the full class name) from a JAR file, but there wasn’t as much information about stuffing a JAR of whatever into the current, running classpath.\u00a0 After much trial and error, this is what I finally produced:<\/p>\n\n\n
\n
\nimport<\/span> java.io.File<\/span>;<\/span>
\nimport<\/span> java.io.FileOutputStream<\/span>;<\/span>
\nimport<\/span> java.io.IOException<\/span>;<\/span>
\nimport<\/span> java.io.InputStream<\/span>;<\/span>
\nimport<\/span> java.net.URL<\/span>;<\/span>
\nimport<\/span> java.nio.channels.Channels<\/span>;<\/span>
\nimport<\/span> java.nio.channels.ReadableByteChannel<\/span>;<\/span>
\nimport<\/span> java.security.KeyManagementException<\/span>;<\/span>
\nimport<\/span> java.security.NoSuchAlgorithmException<\/span>;<\/span>
\nimport<\/span> java.security.cert.CertificateException<\/span>;<\/span>
\nimport<\/span> java.security.cert.X509Certificate<\/span>;<\/span>
\n
\nimport<\/span> javax.net.ssl.HostnameVerifier<\/span>;<\/span>
\nimport<\/span> javax.net.ssl.HttpsURLConnection<\/span>;<\/span>
\nimport<\/span> javax.net.ssl.SSLContext<\/span>;<\/span>
\nimport<\/span> javax.net.ssl.SSLSession<\/span>;<\/span>
\nimport<\/span> javax.net.ssl.TrustManager<\/span>;<\/span>
\nimport<\/span> javax.net.ssl.X509TrustManager<\/span>;<\/span>
\n
\npublic<\/span> class<\/span> UrlDownloader {<\/span>
\n
\n \/**
\n * Downloads the specified URL to the specified file location. Maximum size
\n * allowed is <code>Long.MAX_VALUE<\/code> bytes.
\n *
\n * @param url
\n * location to read
\n * @param file
\n * location to write
\n * @throws NoSuchAlgorithmException
\n * @throws KeyManagementException
\n * @throws IOException
\n *\/<\/span>
\n public<\/span> void<\/span> download(<\/span>URL<\/span><\/a> url, File<\/span><\/a> file)<\/span> throws<\/span> NoSuchAlgorithmException<\/span><\/a>, KeyManagementException<\/span><\/a>, IOException<\/span><\/a> {<\/span>
\n TrustManager [<\/span>]<\/span> trustManagers =<\/span> new<\/span> TrustManager [<\/span>]<\/span> {<\/span> new<\/span> NvbTrustManager(<\/span>)<\/span> }<\/span>;<\/span>
\n final<\/span> SSLContext context =<\/span> SSLContext.getInstance<\/span>(<\/span>"SSL"<\/span>)<\/span>;<\/span>
\n context.init<\/span>(<\/span>null<\/span>, trustManagers, null<\/span>)<\/span>;<\/span>
\n
\n \/\/ Set connections to use lenient TrustManager and HostnameVerifier<\/span>
\n HttpsURLConnection.setDefaultSSLSocketFactory<\/span>(<\/span>context.getSocketFactory<\/span>(<\/span>)<\/span>)<\/span>;<\/span>
\n HttpsURLConnection.setDefaultHostnameVerifier<\/span>(<\/span>new<\/span> NvbHostnameVerifier(<\/span>)<\/span>)<\/span>;<\/span>
\n
\n InputStream<\/span><\/a> is =<\/span> url.openStream<\/span>(<\/span>)<\/span>;<\/span>
\n ReadableByteChannel rbc =<\/span> Channels.newChannel<\/span>(<\/span>is)<\/span>;<\/span>
\n FileOutputStream<\/span><\/a> fos =<\/span> null<\/span>;<\/span>
\n
\n try<\/span> {<\/span>
\n fos =<\/span> new<\/span> FileOutputStream<\/span><\/a>(<\/span>file)<\/span>;<\/span>
\n fos.getChannel<\/span>(<\/span>)<\/span>.transferFrom<\/span>(<\/span>rbc, 0<\/span>, Long<\/span><\/a>.MAX_VALUE<\/span>)<\/span>;<\/span>
\n }<\/span> finally<\/span> {<\/span>
\n if<\/span> (<\/span>fos !=<\/span> null<\/span>)<\/span> {<\/span>
\n fos.close<\/span>(<\/span>)<\/span>;<\/span>
\n }<\/span>
\n is.close<\/span>(<\/span>)<\/span>;<\/span>
\n }<\/span>
\n }<\/span>
\n
\n \/**
\n * Simple <code>TrustManager<\/code> that allows unsigned certificates.
\n *\/<\/span>
\n private<\/span> static<\/span> final<\/span> class<\/span> NvbTrustManager implements<\/span> TrustManager, X509TrustManager {<\/span>
\n @Override
\n public<\/span> void<\/span> checkClientTrusted(<\/span>X509Certificate<\/span><\/a>[<\/span>]<\/span> chain, String<\/span><\/a> authType)<\/span> throws<\/span> CertificateException<\/span><\/a> {<\/span> }<\/span>
\n
\n @Override
\n public<\/span> void<\/span> checkServerTrusted(<\/span>X509Certificate<\/span><\/a>[<\/span>]<\/span> chain, String<\/span><\/a> authType)<\/span> throws<\/span> CertificateException<\/span><\/a> {<\/span> }<\/span>
\n
\n @Override
\n public<\/span> X509Certificate<\/span><\/a>[<\/span>]<\/span> getAcceptedIssuers(<\/span>)<\/span> {<\/span>
\n return<\/span> null<\/span>;<\/span>
\n }<\/span>
\n }<\/span>
\n
\n \/**
\n * Simple <code>HostnameVerifier<\/code> that allows any hostname and session.
\n *\/<\/span>
\n private<\/span> static<\/span> final<\/span> class<\/span> NvbHostnameVerifier implements<\/span> HostnameVerifier {<\/span>
\n @Override
\n public<\/span> boolean<\/span> verify(<\/span>String<\/span><\/a> hostname, SSLSession session)<\/span> {<\/span>
\n return<\/span> true<\/span>;<\/span>
\n }<\/span>
\n }<\/span>
\n}<\/span><\/div><\/div>\n\n<\/pre>\nAdding JAR to classpath<\/h3>\n
\n\n
\n
\nimport<\/span> java.io.File<\/span>