J/Direct

J/Direct was a technology included in some versions of Microsoft Java Virtual Machine, which allowed direct calls into the Windows API. J/Direct was specific of Microsoft's Virtual Machine, in replacement of the standard Java Native Interface (JNI).

A Java program which used J/Direct would not run on platforms other than Microsoft Windows.

The release of J/Direct (along with AFC and WFC), was part of an effort by Microsoft to gain leadership on the growing Java community.^[1]

Since this destroyed one of the main advantages of Java, its cross-platform nature, J/Direct was often seen as an attempt by Microsoft to undermine Java's cross-platform capabilities, and an example of the tactic of embrace, extend and extinguish.^[2]

After the drop by Microsoft of their specific Java virtual machine, and the switch to the .NET environment, J/Direct was no longer maintained.

Architecture

J/Direct used a kind of annotation in Java code to make the link between Java and Windows functions. As annotations did not exist in Java when J/Direct was designed, Microsoft used a special syntax in Java comments.

For example, to declare the GetSysColor function in the User32 API:^[3]

/** @dll.import("USER32", entrypoint="GetSysColor") */
  static native int getSysColor(int nIndex);

The Microsoft Java implementation already provided a pre-defined package which provided a set of pre-defined classes bound to the User32, Gdi32.dll, and Kernel32.dll APIs.^[4]

Additionally, the Microsoft VM used some built-in rules to be able to bind automatically the Java code to some Windows API functions. For example, it chose automatically between ANSI and Unicode versions of Windows API functions.^[5]

See also

• Java Native Interface
• Java Native Access
• P/Invoke, the equivalent .NET API