README for LiveConnect
Author: Scott Furman

This is the README file for the JavaScript LiveConnect Version 3 ("LC3") implementation.

## Table of Contents

> Introduction
> New features
> Compatibility
> Limitations/Bugs/To-Do
> Build conventions
> Naming and coding conventions
> The LiveConnect API
> Sample LiveConnect shell interactions

## Introduction

LiveConnect is a library that permits JavaScript and Java virtual machines to interoperate. Specifically, it enables JavaScript to access Java fields, invoke Java methods and enables Java to access JavaScript object properties and evaluate arbitrary JavaScript. LiveConnect was originally an integrated feature of both the Netscape Navigator browser and Netscape's server-side JavaScript. Now, it is a standalone library that can be embedded within other projects, such as the Mozilla browser. More information on LiveConnect can be found by searching the index on Netscape's DevEdge site. This README assumes basic familiarity with JSRef, the reference implementation of JavaScript, and with the LiveConnect technology.

The JSRef project/makefiles (located in another directory) build a library or DLL containing the JavaScript runtime (compiler, interpreter, decompiler, garbage collector, atom manager, standard classes). The LiveConnect project/makefiles build a library that links with both JSRef and any Java Virtual Machine (JVM) that implements the Java Native Interface (JNI), as specified by JavaSoft. It then compiles a small "shell" example program and links that with the library to make an interpreter that can be used interactively and with test scripts. See the sample shell interactions.

Scott Furman, 10/31/98

## New features

The following features were not available in the versions of LiveConnect that were integrated with Netscape Navigator versions 4.x and earlier. For information on LiveConnect version 1, which was used in Navigator versions 3 and 4, and Enterprise Server 3, see Netscape's DevEdge site or any number of 3rd-party publications.)

#### LiveConnect version 3 (8/31/99)

- In previous versions of LiveConnect, when more than one overloaded Java method was compatible with the types of arguments in an invocation from JS, the choice of Java method was made arbitrarily, by using the first one enumerated by the Java reflection APIs. Unfortunately, the ordering of methods when enumerating is not governed by any specification, so differences between JVM vendors could lead to inconsistencies in LiveConnect behavior. Now, a JVM-independent set of rules is used to choose among a set of overloaded methods. Informally, the method with Java parameter types that most closely match the JavaScript types is chosen.
- The weak correspondence between the JS language typing system and Java's may result in ambiguity and/or shadowing when resolving among overloaded Java methods, even when using LC3's improved method overload resolution algorithm (see above). For example, JS's number type can map to a Java method argument that has any floating-point or integral types: byte, char, short, int, long, float, double. If necessary, it is now possible to bypass the method overload resolution process and explicitly specify the method to be invoked:

`myPrintMethod = java.io.PrintStream["print(double)"];`
`myPrintMethod(13);`

- Static methods can now be invoked using either the class name or a reference to an instance of the class. (Older versions of LiveConnect allow only the former.)
- It is no longer necessary to convert Java Strings to JS strings before using them as the receivers of JS string methods, which is typically done by appending an empty string to the Java string, e.g.

  > `s = new java.lang.String("foo")` // s contains a Java string
  > `s = s + "";` // s is now a JS string
  > `m = s.match(/o?/)`

The explicit conversion to a JS string is no longer required because java.lang.String objects are treated as a special case that "inherit" all the methods of JS strings, i.e. so that the second statement in the example above is now superfluous.

- Similarly, JavaArray objects "inherit" the methods of JS's `Array.prototype`, so it is possible to apply many, though not all, of the JS array utility methods such as `reverse()` and `join()` to JavaArray objects.
- There is now support for the `instanceof` and `in` operators. These operators are currently proposed for inclusion in the ECMA-2 standard.
- LiveConnect has been extended to take advantage of JavaScript exceptions, a language feature that debuted in JavaScript 1.4. Now, when JavaScript calls into Java, any Java exceptions are converted to JS exceptions which can be caught using JS try-catch statements. Similarly, JS exceptions are propagated to Java wrapped in an instance of netscape.javascript.JSException.
- JavaScript Array objects can now be passed to Java methods that expect a Java array as an argument.
  LiveConnect will create a new Java array of the appropriate type with a length equal to that of the JS Array object. Each element of the Java array is filled in by converting the corresponding element of the JS array, including undefined elements, to an equivalent Java value. Note: Since the contents of the JS array are copied, side-effects made by the invoked Java method to the Java array will not be reflected in the JS array argument.

#### LiveConnect version 2 (7/31/98)

> - The Java methods of java.lang.Object are now invokeable methods of `JavaArray` objects, matching the behavior of arrays when accessed from Java. (Java arrays are a subclass of java.lang.Object.) For example, Java's `getClass()` and `hashCode()` methods can now be called on `JavaArray` objects. (In prior versions of LiveConnect, the methods of java.lang.Object were only inherited by non-array Java objects.)

> Note that this change has caused the string representation of JavaArray objects to change. Previously, the JavaArray toString() method always printed "`[object JavaArray]"` for all `JavaArray`'s. Now, the Java `java.lang.Object.toString()` method is called to convert JavaArray objects to strings, just as with other, non-array Java objects that are accessible via LiveConnect. `java.lang.Object.toString()`is defined in the Java Language Specification to return the value of the following expression:

> `getClass().getName() + '@' + Integer.toHexString(hashCode())`

> - A one-character string is now an acceptable match for an argument to a Java method of type `char`. (In earlier versions of LiveConnect, the only acceptable match for a `char` had to be a JavaScript value that was convertible to a number.) For example, the following is now possible:

> `c = new java.lang.Character("F")`

> - A JavaClass object is now an acceptable match for an argument to a Java method of type java.lang.Class. For example, you can now write:

> `java.lang.reflect.Array.newInstance(java.lang.String, 3)`

> instead of the more verbose:

> `jls = java.lang.Class.forName("java.lang.String")`
> `java.lang.reflect.Array.newInstance(jls, 3)`

## Compatibility

Unlike this standalone/component release, all previous versions of LiveConnect appeared only as an integrated feature of Netscape Navigator or the Enterprise Server. The variants of LiveConnect that appeared in Navigator versions 3.x and 4.x all behave much the same, modulo bugs. For brevity we refer to this classic version of LiveConnect as "LC1" (LiveConnect version 1) and this most recent release as "LC3". With a few exceptions LC3 provides a superset of LC1 features. (There was an intermediate LiveConnect release known as "LC2" in 7/98, but it was not used in any products.)

- As in LC1, JavaScript objects appear to Java as instances of netscape.javascript.JSObject. In LC1, two JSObject's could be tested for equality, i.e. to see if they refer to the same instance, by using the `==' operator. Instead, developers must now use the `equals()`method of netscape.javascript.JSObject for comparison, a method that overrides `java.lang.Object.equals()`. Note that using `equals()` instead of `==' will work the same in all versions of LiveConnect, including LC3

[It is not possible to replicate the identity behavior of the `==' operator that LC1 provides without the use of "weak" references, i.e. references that do not contribute to making a Java object reachable for purposes of garbage collection, but which nonetheless allow reference to an object as long as it is reachable by other means. The use of weak references is not portable, however. It is not part of the JNI or JDK 1.1 and it is not provided on all JVMs. The JDK1.2 release includes standard support for weak references.]

- It's possible that, in a set of overloaded Java methods, more than one method is compatible with the types of the actual arguments in a call from JavaScript to Java via LiveConnect. LC1 and LC2 resolved these ambiguities in a simplistic manner, by simply invoking whatever method was enumerated first by the JVM. The enumeration order of reflected methods using java.lang.reflect is not specified by Sun and may differ among vendor's JVMs, i.e. enumeration could be in order of classfile appearance, hashtable order, etc. Hence, the Java method chosen when there is more than one compatible method may vary depending on the JVM. With the Netscape and Sun JVMs, it is possible to change the behavior of an LC1/LC2 program by changing the order that Java methods appear in a source file, thus changing the method enumeration order.

In LC3, a new method overload resolution algorithm is used. Informally, the method with Java parameter types that most closely match the JavaScript types is chosen. You can read all the gorey details in the spec.

- There are several minor changes in error handling to make LiveConnect more conformant to ECMAScript. These include, for example, making any attempt to delete JavaObject, JavaClass or JavaPackage properties fail silently, rather than causing an error. Also, some error messages have been changed to be more informative. These changes should generally be backward-compatible with LC1 because few programs that use LiveConnect will depend on the exact behavior of LiveConnect when handling errors.

## Limitations/Bugs/To-Do

- The efficiency of calling Java methods leaves something to be desired, due to the convoluted nature of implementing native methods for JS. JS_CloneFunctionObject() is called for every Java method invocation and the inability to store private data in a JSFunction object requires that the method table be searched twice instead of once for every invocation.
- When Java objects are referenced from JS, they are entered into a hash table, so as to ensure that the same JS Object wrapper is used every time a particular Java object is reflected into JS. In this way, the behavior of the JS '==' and '===' operators are preserved. Unfortunately, the hash table may grow quite large (objects are only removed from the hash table when finalized). In thread-safe systems, the hash table must be locked when accessed, leading to slow performance. One alternate solution would be to make it possible to overload JS's equality-test operators, so that the hash table would no longer be required.
- Initially, JavaClassDescriptor objectswere reference-counted to permit free'ing of unused descriptors. However, it's relatively common to develop cycles in the graph of JavaClassDescriptors, which leads to unused JavaClassDescriptors that have non-zero reference counts. For that reason, JavaClassDescriptors are not free'ed until JSJ_Shutdown(). Luckily, the amount of memory used by JavaClassDescriptors tends to be relatively small.
- The LiveConnect API is designed to allow multiple JVMs to be used simultaneously in the same executable (although each JSContext is limited to interaction with at most one JVM). However, the API is not fully implemented. For example, many global variables will need to become members of the JSJavaVM struct so that they are stored on a per-JVM basis.
- Java and JavaScript use independent garbage collection systems. A reference between the two worlds must, therefore, take the form of a GC root. It's possible to create uncollectable objects when cyclic graphs cross the boundary between JS and Java, e.g. a JS object that refers to a Java object that refers back to the original JS object. There is no simple solution to this dual-GC problem. Luckily, such cyclic object graphs are extremely rare.

## Build conventions

The following directions are for building the standalone version of LiveConnect. To build the version that's used in the Mozilla browser, see the Mozilla build documentation.

On all platforms, you must update your JVM's `CLASSPATH` to point to the `js/src/liveconnect/classes` subdirectory. If you do not, LiveConnect will still operate but with the limitation that JS objects may not be passed as arguments of Java methods and it will not be possible to call from Java into JavaScript, i.e. the netscape.javascript.JSObject class will be inaccessible. Another downside of operating without these classes is that Java error messages will not include a Java stack trace, when one is available. If your `CLASSPATH` is set improperly, you will see a message like, "`initialization error: Can't load class netscape/javascript/JSObject`" when starting a LiveConnect debug build.

By default, all platforms build a version of LiveConnect that is not threadsafe. If you require thread-safety, you must also populate the `mozilla/dist` directory with NSPR headers and libraries. (NSPR implements a portable threading library, among other things. The source is downloadable via CVS from `mozilla/nsprpub`.) Next, you must define `JS_THREADSAFE` when building LiveConnect, either on the command-line (gmake/nmake) or in a universal header file. Note that JSRef must also be built with `JS_THREADSAFE`.

One important note about building on Windows: There are two independent build systems (in addition to the Mozilla browser build system). One of them uses the IDE project files and the other uses gmake and makefiles. The former will be preferred by most for debugging and the latter is more complete, since it builds the necessary Java classes in addition to compiling the LiveConnect C code.

Windows

- Build the JS runtime and interpreter, `js32.dll`, by using the normal JSRef build procedure.
- Set the `JDK` environment variable to point to the top-level JDK directory, e.g. `D:\jdk1.1.5`. This is used to establish paths for header file inclusion, linking and execution. If you are not using Sun's JVM, the project files may require manual tweaking to set these paths correctly.
- Use MSVC 5 or MSVC 6 with the `LiveConnectShell.dsw` project file. NOTE: makefile.win is an nmake file used only for building the JS-engine in the Mozilla browser. Don't attempt to use it to build the standalone JS-engine.
- The output files (DLLs and executables) are placed in either the `js\src\liveconnect\Debug` or the `js\src\liveconnect\Release` directory.
- The LiveConnect-enabled shell is named `lcshell.exe` and appears in the output directory.
- You must have the JVM DLL in your `PATH` environment variable in order to run. If you are using the Sun JDK, the DLL appears in the JDK's bin directory, e.g. `D:\jdk1.1.5\bin\javai_g.dll`.
- Use any Java compiler to compile the java source files in the `js\src\liveconnect\classes\netscape\javascript` directory.
- Update your JVM's `CLASSPATH` to point to the `js\src\liveconnect\classes` subdirectory. (See above)

Mac OS

- Using CodeWarrior Pro 3 is recommended, though the project files will probably also work with CodeWarrior Pro 4.
- Install Apple's JVM, MRJ 2.0 (or later), and the MRJ SDK v2.0.1ea4. Note: You do not need to install MRJ if you are running a recent version of MacOS 8, since it is shipped with the OS.
- Copy the folders `CIncludes` & `Libraries` from the SDK's `Interfaces&Libraries` directory to `js:src:liveconnect:macbuild:JavaSession`.
- Build the LiveConnect test application, `LiveConnectShell`, with `js:src:liveconnect:macbuild:LiveConnectShell.mcp`.
- Build `liveconnect.jar` with `js:src:liveconnect:macbuild:LiveConnect.mcp`.
- Make an alias to `liveconnect.jar` and place it in "`{SystemFolder}Extensions:MRJ Libraries:MRJClasses`".

Unix (also works on Windows)

- Use '`gmake -f Makefile.ref`' to build. To compile optimized code, pass `BUILD_OPT=1` on the gmake command line or preset it in the environment or `Makefile.ref`. NOTE: Do not attempt to use `Makefile` to build. This file is used only for building LiveConnect in the Mozilla browser.
- Each platform on which LiveConnect is built must have a *.mk configuration file in the `js/src/liveconnect/config` directory. The configuration file specifies the JVM headers/libraries used and allows for customization of command-line options. To date, the build system has been tested on Solaris, AIX, HP/UX, OSF, IRIX, x86 Linux and Windows NT. Most platforms will work with either the vendor compiler or gcc.
- Update your JVM's `CLASSPATH` to point to the `js/src/liveconnect/classes` subdirectory. (See above)

## Naming and coding conventions:

- Public function names begin with JSJ_ followed by capitalized "intercaps", e.g. JSJ_ConnectToJavaVM.
- Extern but library-private function names use a jsj_ prefix and mixed case, e.g. jsj_LookupSymbol.
- Most static function names have unprefixed, underscore-separated names: get_char.
- But static native methods of JS objects have intercaps names, e.g., JavaObject_getProperty().
- And library-private and static data use underscores, not intercaps (but library-private data do use a js_ prefix).
- Scalar type names are lowercase and js-prefixed: jsdouble.
- Aggregate type names are JS-prefixed and mixed-case: JSObject.
- Macros are generally ALL_CAPS and underscored, to call out potential side effects, multiple uses of a formal argument, etc.
- Four spaces of indentation per statement nesting level. The files are space-filled, so adjusting of your tab setting should be unnecessary.
- I don't bow down to the ancient "80 columns per line" gods, since most of us are not using vt100's to read source code. My rule of thumb is to use no more than 95 columns per line, but exceptions are made to format tables or table-like code.
- DLL entry points have their return type expanded within a JS_EXPORT_API() macro call, to get the right Windows secret type qualifiers in the right places for both 16- and 32-bit builds.
- Callback functions that might be called from a DLL are similarly macroized with JS_STATIC_DLL_CALLBACK (if the function otherwise would be static to hide its name) or JS_DLL_CALLBACK (this macro takes no type argument; it should be used after the return type and before the function name).

## The LiveConnect API

All public LiveConnect entry points and callbacks are documented in jsjava.h, the header file that exports those functions.

## File walk-through

jsjava.h | LiveConnect's only public header file. Defines all public API entry points, callbacks and types.
jsj_private.h | LiveConnect internal header file for intra-module sharing of functions and types.
jsj.c | Public LiveConnect API entry points and initialization code. Handling of multiple threads and multiple JVMs.
jsj_array.c | Read and write elements of a Java array, performing needed conversions to/from JS types.
jsj_class.c | Construct and manipulate JavaClassDescriptor structs, which are the native representation for Java classes. JavaClassDescriptors are used to describe the methods and fields of a class, including their type signatures, and include a reference to the peer java.lang.Class object. Since each Java object has a class, there is a JavaClassDescriptor associated with the JavaScript reflection of each Java Object.
jsj_convert.c | Convert between Java and JavaScript values of all types, which may require calling routines in other files to wrap JS objects as Java objects and vice-versa.
jsj_field.c | Reflect Java fields as properties of JavaObject objects and implement getter/setter access to those fields.
jsj_JavaArray.c | Implementation of the JavaScript JavaArray class. Instances of JavaArray are used to reflect Java arrays.
jsj_JavaClass.c | Implementation of the JavaScript JavaClass class. Instances of JavaClass are used to reflect Java classes.
jsj_JavaObject.c | Implementation of the JavaScript JavaObject class. Instances of JavaObject are used to reflect Java objects, except for Java arrays, although some of the code in this file is used by the JavaArray code.
jsj_JavaMember.c | Implementation of the JavaScript JavaMember class. JavaMember's are a strange beast required only to handle the special case of a public field and a public method that appear in the same Java class and which have the same name.
jsj_JavaPackage.c | Implementation of the JavaScript JavaPackage class. Instances of JavaPackage are used to reflect Java packages. The JS properties of a JavaPackage are either nested JavaPackage objects or a JavaClass object.
jsj_JSObject.c | Implementation of the native methods for the netscape.javascript.JSObject Java class, which are used for calling into JavaScript from Java. It also contains the code that wraps JS objects as instances of netscape.javascript.JSObject and the code that handles propagation of exceptions both into and out of Java.
jsj_method.c | Reflect Java methods as properties of JavaObject objects and make it possible to invoke those methods. Includes overloaded method resolution and argument/return-value conversion code.
jsj_utils.c | Low-level utility code for reporting errors, etc.

## Sample LiveConnect shell interactions

#### Java packages, classes and constructors

`js> java`
`[JavaPackage java]`
`js> awt = java.awt`
`[JavaPackage java.awt]`
`js> Rectangle = awt.Rectangle`
`[JavaClass java.awt.Rectangle]`

#### Java instance fields and methods

`js> r = new java.awt.Rectangle(34, 23)`
`java.awt.Rectangle[x=0,y=0,width=34,height=23]`
`js> r.width - r.height`
`11`
`js> r.x = 7; r.y = 4`
`4`
`js> r`
`java.awt.Rectangle[x=7,y=4,width=34,height=23]`
`js> r.grow(3)`
`There is no Java method java.awt.Rectangle.grow that matches JavaScript argument types (number).`
`Candidate methods with the same name are:`
`void grow(int, int)`

`js> r.grow(3, 3)`
`js> r`
`java.awt.Rectangle[x=4,y=1,width=40,height=29]`

#### Java arrays

`js> s = new java.lang.String("mastiff")`
`mastiff`
`js> c = s.toCharArray()`
`[C@298e9b`
`js> c[0] = "b"; c[4] = "a"; c[5] = "r"; c[6] = "d"`
`d`
`js> s2 = new java.lang.String(c)`
`bastard`

#### Java static fields and methods

`js> java.lang.reflect.Modifier.ABSTRACT`
`1024`
`js> java.lang.Math.sin(3) + 2`
`2.1411200080598674`

#### Explicit resolution of overloaded Java methods

`js> x = "23"`
`23`
`js> java.lang.Math.abs(x)`
`The choice of static Java method java.lang.Math.abs matching JavaScript argument types (string) is ambiguous.`
`Candidate methods are:`
`long abs(long)`
`float abs(float)`
`double abs(double)`
`int abs(int)`
`js> abs = java.lang.Math["abs(int)"]`

`function abs(int)() {`
`[native code]`
`}`

`js> abs(x)`
`23`

#### Public Method/field enumeration

`js> out = java.lang.System.out`
`java.io.PrintStream@2980f5`
`js> for (m in out) print(m)`
`println`
`print`
`checkError`
`close`
`flush`
`write`
`wait`
`notifyAll`
`notify`
`toString`
`equals`
`hashCode`
`getClass`
`js> for (m in java.lang.String) print(m)`
`copyValueOf`
`valueOf`

#### 'instanceof' and 'in' operators

`js> s = new java.lang.String("foop")`
`foop`
`js> s instanceof java.lang.Class`
`false`
`js> s instanceof java.lang.Object`
`true`
`js> "valueOf" in s`
`true`
`js> "NoSuchThing" in s`
`false`

#### Applying JavaScript string methods to Java strings

`js> s = new java.lang.String("The rain in Spain falls mainly on my head.")`
`The rain in Spain falls mainly on my head.`
`js> s.match(/Spain.*my/)`
`Spain falls mainly on my`

#### Applying JavaScript array methods to Java arrays

`js> s = new java.lang.String("JavaScript")`
`JavaScript`
`js> c = s.toCharArray()`
`[C@298aef`
`js> c.reverse()`
`[C@298aef`
`js> new java.lang.String(c)`
`tpircSavaJ`