无法从Process.StandardOutput中读取

Question

I am trying to use a Process to check the version of Java installed using the "-version" command line parameter. However when I try to read the command line output from the java application I get a null value.

My code is fairly simple:

Process java = new Process();
java.StartInfo.RedirectStandardOutput = true;
java.StartInfo.CreateNoWindow = true;
java.StartInfo.UseShellExecute = false;
java.StartInfo.FileName = "java";
java.StartInfo.Arguments = "-version";
java.Start();

// version comes out null
string version = java.StandardOutput.ReadLine();

// There are probably better ways to extract this data but
// I want to get it working before I cross that bridge.
string versionNumber = version.Substring(16, 1);
hasJava = int.Parse(versionNumber) < 7;

java.Close();

It is probably something small and easy, but I can't see it.

Answer 1

You can read the currently installed Java version from the registry as well. The following file includes the method TryGetJavaHome , which supports 32- and 64-bit virtual machines for specific vendor/installation combinations.

bool TryGetJavaHome(
    RegistryView registryView,
    string vendor,
    string installation,
    out string javaHome)

Vendors:

• "JavaSoft" (HotSpot)
• "JRockit"

Installations:

• "Java Runtime Environment"
• "Java Development Kit"

If you look at the code for the method, you'll find the following line which obtains the current version.

object currentVersion = javaKey.GetValue("CurrentVersion");

This will return a value like "1.7" or "1.6". If want a more complete version number, some installations provide the following.

object familyVersion6 = javaKey.GetValue("Java6FamilyVersion");
object familyVersion7 = javaKey.GetValue("Java7FamilyVersion");

For some installations, the family version is not present, but for the standard runtime installation I see the values "1.6.0_38" and "1.7.0_13".

Reference: Antlr4ClassGenerationTaskInternal.cs

Answer 2

If you remove java.CreateNoWindow = true; , it works.

Answer 3

After some testing, I've found that the information you're looking for, for some reason, can be found in the standardError stream.

try the following.

Process java = new Process();
java.StartInfo.RedirectStandardError = true;
java.StartInfo.CreateNoWindow = true;
java.StartInfo.UseShellExecute = false;
java.StartInfo.FileName = "java";
java.StartInfo.Arguments = "-version";
java.Start();

// version comes out null
string version = java.StandardError.ReadLine();

// There are probably better ways to extract this data but
// I want to get it working before I cross that bridge.
string versionNumber = version.Substring(16, 1);

java.Close();