Question

How to get the Eclipse installation directory through programming in swt/java.
I actually want to get the plugins directory of the eclipse.

Answer 1

Update (January 2012)
James Moore mentions in the comments that the FAQ and API are quite old.

FileLocator.resolve(URL) is now preferred to the deprecated Platform.resolve().

As this example shows, you need to pass the actual resource (here a bundle), not the name of the resource, in order to resolve it:

private static URI locateFile(String bundle, String fullPath) {
  try {
    URL url = FileLocator.find(Platform.getBundle(bundle), new Path(fullPath), null);
    if(url != null)
      return FileLocator.resolve(url).toURI();
  } catch (Exception e) {}
    return null;
  }
}

See also “How to refer a file from jar file in eclipse plugin” for more.

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Original answer (January 2011)

Maybe the FAQ “How do I find out the install location of a plug-in?” can help here:

You should generally avoid making assumptions about the location of a plug-in at runtime.
To find resources, such as images, that are stored in your plug-in’s install directory, you can use URLs provided by the Platform class. These URLs use a special Eclipse Platform protocol, but if you are using them only to read files, it does not matter.

In Eclipse 3.1 and earlier, the following snippet opens an input stream on a file called sample.gif located in a subdirectory, called icons, of a plug-in’s install directory:

  Bundle bundle = Platform.getBundle(yourPluginId);
  Path path = new Path("icons/sample.gif");
  URL fileURL = Platform.find(bundle, path);
  InputStream in = fileURL.openStream();

If you need to know the file system location of a plug-in, you need to use Platform.resolve(URL). This method converts a platform URL to a standard URL protocol, such as HyperText Transfer Protocol (HTTP), or file.
Note that the Eclipse Platform does not specify that plug-ins must exist in the local file system, so you cannot rely on this method’s returning a file system URL under all circumstances in the future.

In Eclipse 3.2, the preferred method seems to be:

  Bundle bundle = Platform.getBundle(yourPluginId);
  Path path = new Path("icons/sample.gif");
  URL fileURL = FileLocator.find(bundle, path, null);
  InputStream in = fileURL.openStream();