LJAPI Overview¶

Static Method Code¶

local(class) = java_jvm_getenv->FindClass('java/lang/Math')
local(mID)   = java_jvm_getenv->GetStaticMethodID(#class, 'scalb', '(FI)F')

java_jvm_getenv->CallStaticFloatMethod(#class, #mID, jfloat(4.0), jint(3))

// => 32.000000

Static Method Walkthrough¶

One thing to notice is that all the communication is done using java_jvm_getenv. This method returns the java_jnienv object for the Lasso instance, and it is this object that allows Lasso to communicate with the Java Virtual Machine (JVM).

1. The first line of code finds the Java class we want to work with and returns a Lasso jobject; storing it into the local variable “class”. The string value that gets passed to FindClass is the fully qualified class name signature (or array type signature). For more information, see the FindClass documentation.

   local(class) = java_jvm_getenv->FindClass('java/lang/Math')
   

2. The next line of code looks up the method ID for the method we want to execute and returns it as a jmethodid type, storing it into the “mID” variable. GetStaticMethodID takes in the class (jobject) object we found in the first line, the name of the method as the second parameter, and the signature for that method as the third parameter. For more information, see the GetStaticMethodID documentation.

   local(mID)   = java_jvm_getenv->GetStaticMethodID(#class, 'scalb', '(FI)F')
   

3. The method signature (FI)F specifies that it takes a float and an int parameter and returns a float. The easiest way to find the signature for a method is to use the javap command on the command line. In the example below, we run javap -s -p java.lang.Math to get all the method signatures found in the “java.lang.Math” class, and we use grep to filter and find the “scalb” method. You’ll notice in the result that there are actually two methods with the same name but with different signatures, and we’re using the second one:

   $> javap -s -p java.lang.Math | grep -A 1 scalb
   public static double scalb(double, int);
     Signature: (DI)D
   --
   public static float scalb(float, int);
     Signature: (FI)F
   

4. Finally, we execute the method using CallStaticFloatMethod which takes in the class object from the first step and the method ID from the second step and then the required parameters for the method we are calling, if any. Note that we must convert Lasso decimal objects to jfloat and Lasso integer objects to jint.

   java_jvm_getenv->CallStaticFloatMethod(#class, #mID, jfloat(4.0), jint(3))
   

Java Object Member Method Code¶

local(class) = java_jvm_getenv->FindClass('java/util/zip/ZipFile')
local(mID)   = java_jvm_getenv->GetMethodID(#class, '<init>', '(Ljava/lang/String;)V')
local(obj)   = java_jvm_getenv->NewObject(#class, #mID, '/path/to/zipfile.zip')

local(class) = java_jvm_getenv->GetObjectClass(#obj)
local(mID)   = java_jvm_getenv->GetMethodID(#class, 'size', '()I')

java_jvm_getenv->CallIntMethod(#obj, #mID)

// => 92

Java Object Member Method Walkthrough¶

Once again, all the communication is done using the java_jvm_getenv method, which wraps the Lasso instance’s java_jnienv object.

1. The first line of code gets the specified Java class and stores a Lasso jobject into the local variable “class”. The value that gets passed to FindClass is the fully qualified class name signature (or array type signature). For more information, see the FindClass documentation.

   local(class) = java_jvm_getenv->FindClass('java/util/zip/ZipFile')
   

2. Next, the code finds the method ID for the constructor method by passing the class object we found in the first step, “<init>” for the method name, and the method signature as the third argument:

   local(mID)   = java_jvm_getenv->GetMethodID(#class, '<init>', '(Ljava/lang/String;)V')
   

3. The method signature (Ljava/lang/String;)V specifies that it takes a string parameter and returns “void”. The easiest way to find the signature for a method is to use the javap command on the command line. In the example below, we run javap -s -p java.util.zip.ZipFile to get all the method signatures found in the “java.util.zip.ZipFile” class, and we use grep to filter and find the constructor methods. You’ll notice in the result that there are actually three constructor methods, each with different signatures, and we are using the first one:

   $> javap -s -p java.util.zip.ZipFile | grep -A 1 "public java.util.zip.ZipFile"
   public java.util.zip.ZipFile(java.lang.String) throws java.io.IOException;
     Signature: (Ljava/lang/String;)V
   --
   public java.util.zip.ZipFile(java.io.File, int) throws java.io.IOException;
     Signature: (Ljava/io/File;I)V
   --
   public java.util.zip.ZipFile(java.io.File) throws java.util.zip.ZipException, java.io.IOException;
     Signature: (Ljava/io/File;)V
   

4. After finding the constructor method for our class, the code instantiates an object by passing that information into NewObject. The line of code below stores a Java object into “obj” by calling NewObject with the class information, method ID, and any additional parameters required by the constructor (in this case, the path to the zipped file). For more information on NewObject, see the NewObject documentation.

   local(obj)   = java_jvm_getenv->NewObject(#class, #mID, '/path/to/zipfile.zip')
   

5. The next line isn’t actually necessary since the “class” variable already has the class information for “java.util.zip.ZipFile”, but we have it here to demonstrate how you could deal with wanting to call methods on Java objects that were returned by other methods. So, GetObjectClass returns the class information for the specified object. For more information, see the GetObjectClass documentation.

   local(class) = java_jvm_getenv->GetObjectClass(#obj)
   

6. The next line gets the method ID for the size member method and stores it in the local variable “mID”:

   local(mID)   = java_jvm_getenv->GetMethodID(#class, 'size', '()I')
   

7. Finally, we execute the size member method by calling CallIntMethod with the Java object as the first parameter and the method ID for size as the second parameter. Notice that the return type (int) is in the name of the method. There are a number of these methods for various return types.

   java_jvm_getenv->CallIntMethod(#obj, #mID)