gobind

Overview ¶

Gobind generates language bindings that make it possible to call Go functions from Java and Objective-C.

Typically gobind is not used directly. Instead, a binding is generated and automatically packaged for Android or iOS by `gomobile bind`. For more details on installing and using the gomobile tool, see https://golang.org/x/mobile/cmd/gomobile.

Binding Go ¶

Gobind generates target language (Java or Objective-C) bindings for each exported symbol in a Go package. The Go package you choose to bind defines a cross-language interface.

Bindings require additional Go code be generated, so using gobind manually requires calling it twice, first with -lang=<target>, where target is either java or objc, and again with -lang=go. The generated package can then be _ imported into a Go program, typically built with -buildmode=c-archive for iOS or -buildmode=c-shared for Android. These details are handled by the `gomobile bind` command.

Passing Go objects to target languages ¶

Consider a type for counting:

package mypkg

type Counter struct {
	Value int
}

func (c *Counter) Inc() { c.Value++ }

func NewCounter() *Counter { return &Counter{ 5 } }

In Java, the generated bindings are,

public abstract class Mypkg {
	public static native Counter newCounter();
}

and

public final class Counter {
	public Counter() { ... }

	public final long getValue();
	public final void setValue(long v);
	public void inc();

}

The package-level function newCounter can be called like so:

Counter c = Mypkg.newCounter()

For convenience, functions on the form NewT(...) *T are converted to constructors for T:

Counter c = new Counter()

Both forms returns a Java Counter, which is a proxy for a Go *Counter. Calling the inc, getValue and setValue methods will call the Go implementations of these methods.

Similarly, the same Go package will generate the Objective-C interface

@class GoMypkgCounter;

@interface GoMypkgCounter : NSObject {
}

@property(strong, readonly) id ref;
- (void)inc;
- (int64_t)value;
- (void)setValue:(int64_t)v;
@end

FOUNDATION_EXPORT GoMypkgCounter* GoMypkgNewCounter(void);

The equivalent of calling newCounter in Go is GoMypkgNewCounter in Objective-C. The returned GoMypkgCounter* holds a reference to an underlying Go *Counter.

Passing target language objects to Go ¶

For a Go interface:

package myfmt

type Printer interface {
	Print(s string)
}

func PrintHello(p Printer) {
	p.Print("Hello, World!")
}

gobind generates a Java interface that can be used to implement a Printer:

public abstract class Myfmt {
	public static void printHello(Printer p0);
}

and

public interface Printer {
	public void print(String s);
}

You can implement Printer, and pass it to Go using the printHello package function:

public class SysPrint implements Printer {
	public void print(String s) {
		System.out.println(s);
	}
}

The Java implementation can be used like so:

Printer printer = new SysPrint();
Myfmt.printHello(printer);

For Objective-C binding, gobind generates a protocol that declares methods corresponding to Go interface's methods.

@protocol GoMyfmtPrinter
- (void)Print:(NSString*)s;
@end

FOUNDATION_EXPORT void GoMyfmtPrintHello(id<GoMyfmtPrinter> p0);

Any Objective-C classes conforming to the GoMyfmtPrinter protocol can be passed to Go using the GoMyfmtPrintHello function:

@interface SysPrint : NSObject<GoMyfmtPrinter> {
}
@end

@implementation SysPrint {
}
- (void)Print:(NSString*)s {
	NSLog("%@", s);
}
@end

The Objective-C implementation can be used like so:

SysPrint* printer = [[SysPrint alloc] init];
GoMyfmtPrintHello(printer);

Type restrictions ¶

At present, only a subset of Go types are supported.

All exported symbols in the package must have types that are supported. Supported types include:

• Signed integer and floating point types.

• String and boolean types.

• Byte slice types. Note that byte slices are passed by reference, and support mutation.

• Any function type all of whose parameters and results have supported types. Functions must return either no results, one result, or two results where the type of the second is the built-in 'error' type.

• Any interface type, all of whose exported methods have supported function types.

• Any struct type, all of whose exported methods have supported function types and all of whose exported fields have supported types.

Unexported symbols have no effect on the cross-language interface, and as such are not restricted.

The set of supported types will eventually be expanded to cover more Go types, but this is a work in progress.

Exceptions and panics are not yet supported. If either pass a language boundary, the program will exit.

Reverse bindings ¶

Gobind also supports accessing API from Java or Objective C from Go. Similar to how Cgo supports the magic "C" import, gobind recognizes import statements that start with "Java/" or "ObjC/". For example, to import java.lang.System and call the static method currentTimeMillis:

import "Java/java/lang/System"

t := System.CurrentTimeMillis()

Similarly, to import NSDate and call the static method [NSDate date]:

import "ObjC/Foundation/NSDate"

d := NSDate.Date()

Gobind also supports specifying particular classes, interfaces or protocols a particular Go struct should extend or implement. For example, to create an Android Activity subclass MainActivity:

import "Java/android/app/Activity"

type MainActivity struct {
	app.Activity
}

Gobind also recognizes Java interfaces as well as Objective C classes and protocols the same way.

For more details on binding the native API, see the design proposals, https://golang.org/issues/16876 (Java) and https://golang.org/issues/17102 (Objective C).

Avoid reference cycles ¶

The language bindings maintain a reference to each object that has been proxied. When a proxy object becomes unreachable, its finalizer reports this fact to the object's native side, so that the reference can be removed, potentially allowing the object to be reclaimed by its native garbage collector. The mechanism is symmetric.

However, it is possible to create a reference cycle between Go and objects in target languages, via proxies, meaning objects cannot be collected. This causes a memory leak.

For example, in Java: if a Go object G holds a reference to the Go proxy of a Java object J, and J holds a reference to the Java proxy of G, then the language bindings on each side must keep G and J live even if they are otherwise unreachable.

We recommend that implementations of foreign interfaces do not hold references to proxies of objects. That is: if you implement a Go interface in Java, do not store an instance of Seq.Object inside it.

Further reading ¶

Examples can be found in http://golang.org/x/mobile/example.

Design doc: http://golang.org/s/gobind