Invoke Methods - Mule 4

Java methods (either instance or static) can be called through the invoke and invoke static operations in the Java module. Their return value is placed in the payload of the output message or can be placed in a target variable.

To get a detailed description of the configurable parameters for the Java Invoke operation, review the Java module reference operations section.

Invoke Instance Methods

In the following Java class TaxCalculator, belongs to the com.me package:

package com.me;

public class TaxCalculator {

    private Double percentBaseTax = 7.0;

    public Double calculateTax(Double price, Integer percentAdditionalTax) {
        return price * (this.percentBaseTax + percentAdditionalTax) / 100;
    }

    public Boolean isTaxFree(Double price) {
        if (price < 10) {
            return true;
        }
        return false;
    }
}

Unless the classes used with the Java module belong to the default package, the classes must be exported; otherwise, execution fails with a JAVA:CLASS_NOT_FOUND error. See how to export resources.

To invoke instance methods:

Use the new operation from the Java module to create an object on which a method is later invoked. This provides an instanced object that can then call one of its methods.
Set the invoke operation to call one of the object’s methods. Just like the new operation, invoke takes a map for input parameters (if the method has them) and supports target parameters.

In the next example, an instance of the TaxCalculator class is created and placed into the taxCalculator target variable. Then the calculateTax(Double, Integer) method is called, which takes price and percentAdditionalTax as arguments, and its return value is placed in the totalTax variable.

<java:new class="com.me.TaxCalculator"
	constructor="TaxCalculator()"
	target="taxCalculator"/>

<java:invoke instance="#[vars.taxCalculator]"
  class="com.me.TaxCalculator"
  method="calculateTax(Double, Integer)"
  target="totalTax">
      <java:args>#[{
        price: 25.5,
        percentAdditionalTax: 2
      }]</java:args>
</java:invoke>

For the method parameters, the full package name can be specified, for example constructor="Person(java.lang.Double, java.lang.Integer)". This is not needed, but it can be useful to add more clarity in the code or in the case there are clashing class names in the Java code.

In Anypoint Studio, the Java module supports DataSense for the invoke operation, which provides metadata for both the input arguments and the output value.

In the example, DataSense discovers that calculateTax returns a Number, so the output metadata for the invoke operation looks like:

For a complete example of using invoke in Studio, see New and Invoke Operations in Studio.

When configuring the constructor arguments in the args parameter, the keys of the map determine how the parameters are passed to the constructor.

To reference the parameters by name (price, percentAdditionalTax, etc.), the Java class containing the method or constructor has to be compiled using the -parameters compiler flag. If the class was not compiled with this flag, the same parameters must be referenced in the declared order and with the canonical names (arg0, arg1, etc.).

In this case:

<java:args>#[{
  arg0: 25.5,
  arg1: 2
}]</java:args>

If the Java classes are defined in a Studio project, the Maven compiler plugin must be configured in the pom.xml to compile Java classes with the -parameters flag:

<plugin>
    <groupId>org.apache.maven.plugins</groupId>
    <artifactId>maven-compiler-plugin</artifactId>
    <version>3.8.1</version>
    <configuration>
        <compilerArgs>
            <arg>-parameters</arg>
        </compilerArgs>
    </configuration>
</plugin>

Invoke Methods with DataWeave

The Java module also has a DataWeave function (Java::invoke) to provide the same functionality as the invoke operation but inside a DataWeave expression. This practice is especially helpful for methods that return boolean values. The function takes as arguments the full class name of an object, the instance method to execute, the instanced object, and the method arguments as a map.

In the Java class in Invoke Instance Methods, this examples creates a new instance of TaxCalculator and, rather than use the Java module invoke operation to call the isTaxFree(Double) method, it embeds the DataWeave function in a Choice component:

<java:new class="com.foo.TaxCalculator"
	constructor="TaxCalculator()"
	target="taxCalculator"/>

<choice>
    <when
      expression="#[Java::invoke('com.foo.TaxCalculator', 'isTaxFree(Double)', vars.taxCalculator, {price: vars.price})]">
        <flow-ref name="skipTax" />
    </when>
</choice>

Invoke Static Methods

An example on how to invoke static Java methods:

<java:invoke-static
	class="java.lang.Math"
	method="toRadians(double)">
    <java:args>#[{
        arg0: 180
    }]</java:args>
</java:invoke-static>

New and Invoke Operations in Studio

In Anypoint Studio, you can write or load a Java package into a project, configure Java operations within one or more a flows, and run the Mule app in which the operations reside.

This example task sets up two flows in a Mule app:

Write a small Java package: com.examples.
Write two simple Java classes that contain instance methods: Hello.java and Add.java.
Use the Apache Maven compiler plug-in to compile the classes in a way that allows for the use of named parameters in your configurations, instead of arg0, arg1, and so on.
Use the New operation in the Java module to instantiate the Hello and Add objects and to access named parameters in the instance methods.
Use the Invoke operation in the Java module to invoke the hello() and add(3,4) instance methods.
Run the Mule app to execute the New and Invoke operations in Studio.

Assume that you want to invoke methods in two simple Java classes:

Hello class with an instance method

package com.examples;

public class Hello {

  //Constructor
  public Hello() { }

  //Instance method: hello()
  //Returns the string "helloWorld".
  public String hello() {
    return "helloWorld";
  }
}

Add a class with a parameterized instance method

public class Add {

  public int x = 0;
  public int y = 0;

  //Constructor
  public Add(int numA, int numB) {
	        x = numA;
	        y = numB;
  }

  //Instance method with parameters: add(int x, int y).
  //Returns the sum of inputs x and y.
  public int add(int x, int y) {
    return x + y;
  }
}

To create a Mule app that invokes hello() and add():

In Studio, select File > New > Project, provide a project name (javaexamples), and click Finish.
Create a Java package for your classes by right-clicking your Mule project’s src/main/java directory in Package Explorer.
Select New > Package.
Provide the package name com.examples in the Name field.
Click Finish.

Make sure that the package com.examples appears under the src/main/java directory.
Add the Java code for the Hello and Add classes to your new com.examples package by right-clicking your new com.examples package in Studio, and select New > Class.
In the Name field, type Hello.java for the Hello class, and click Finish.

Copy and paste the Hello class content into the Hello.java file from the listing that follows.

The entire file looks like this, including package com.examples at the top:

package com.examples;

public class Hello {

  //Constructor
  public Hello() { }

  //Instance method: hello()
  //Returns the string "helloWorld".
  public String hello() {
    return "helloWorld";
  }
}

Right-click your new com.examples package in Studio, and select New > Class.
In the Name field, typ the Add.java value.
Click Finish.

Copy and paste the class content into the Add.java file.