Elvis Operator for Java (Java Compiler plugin)

One way of writing null safe code in Java is using Optional.ofNullable, Even though I feel it is a better alternative to writing if it gets repetitive and verbose.

I have written Optional.ofNullable countless number of times, and I don't want to write it anymore :)

This project aims to mitigate those problems using Java Compiler Plugin API.

Usage

Consider you need to fetch cardNumber from order object, given order and its nested objects can be null, code snippet would look something like below

Java 7

public String getCardNumberFromOrder(Order order) {
    if(order != null 
            && order.getPayment() != null
            && order.getPayment().getCard() != null) {
        return order.getPayment().getCard().getCardNumber();
    }
    return null;
}

Java 8

Functional way with Java 8

public String getCardNumberFromOrder(Order order) {
    return Optional.ofNullable(order)
        .map(Order::getPayment)
        .map(Payment::getCard)
        .map(Card::getCardNumber)
        .orElse(null);
}

Elvis Plugin

public String getCardNumberFromOrder(Order order) {
    @NullSafe
    String cardNumber = order.getPayment().getCard().getCardNumber();
    return cardNumber;
}

If you open class file for this code it will look something like below

public String getCardNumberFromOrder(Order order) {
    return Optional.ofNullable(order)
        .map(it -> it.getPayment())
        .map(it -> it.getCard())
        .map(it -> it.getCardNumber())
        .orElse(null);
}

This is done using AST manipulation during compile time.

If you want to assign default value to cardNumber in above example, then code will look something like below

public String getCardNumberFromOrder(Order order) {
    @NullSafe("1111222233334444")
    String cardNumber = order.getPayment().getCard().getCardNumber();
    return cardNumber;
}

For more examples please refer here and here .

Installation

Maven configuration

You need to add -Xplugin:ElvisPlugin in compiler arguments

<dependencies>
    <dependency>
        <groupId>io.github.vinayalodha</groupId>
        <artifactId>elvis-plugin</artifactId>
        <version>1.0.3</version>
        <scope>provided</scope>
    </dependency>
</dependencies>
<plugin>
    <groupId>org.apache.maven.plugins</groupId>
        <artifactId>maven-compiler-plugin</artifactId>
        <version>3.8.1</version>
        <configuration>
            <source>11</source>
            <target>11</target>
            <compilerArgs>
                <arg>-Xplugin:ElvisPlugin</arg>
            </compilerArgs>
    </configuration>
</plugin>

For JDK 16 you need to set <fork>true</fork> and add additional args in tag. Please refer to JSR

<arg>-J--add-exports=jdk.compiler/com.sun.tools.javac.api=ALL-UNNAMED</arg>
<arg>-J--add-exports=jdk.compiler/com.sun.tools.javac.code=ALL-UNNAMED</arg>
<arg>-J--add-exports=jdk.compiler/com.sun.tools.javac.model=ALL-UNNAMED</arg>
<arg>-J--add-exports=jdk.compiler/com.sun.tools.javac.tree=ALL-UNNAMED</arg>
<arg>-J--add-exports=jdk.compiler/com.sun.tools.javac.util=ALL-UNNAMED</arg>

IntelliJ Idea

You need to add -Xplugin:ElvisPlugin in compiler arguments

Current limitations

Works only on local variable

Currently, @NotNull can only be used on local variable, this is due to fact Java annotation limit target type for annotation to local variable

for example

public String getCardNumberFromOrder(Order order) {
    @NullSafe
    String cardNumber = order.getPayment().getCard().getCardNumber();
    return cardNumber;
}

can not be written as

public String getCardNumberFromOrder(Order order) {
    @NullSafe
    return order.getPayment().getCard().getCardNumber(); // In Java you cant have annotation on return statement
}

If you have classname in expression then you will get compilation error

For example, below code will give compilation error, given it contain classname(StringUtils) in expression

public String getCardNumberFromOrder(Order order) {
    @NullSafe
    String cardNumber = StringUtils.INSTANCE.trim();
    return cardNumber;
}

Above code will throw compilation error saying Unable to find Symbol StringUtils

If expression have Checked exception it will give compilation error

Consider example

public String getSomeStuff(SomeClass object) {
    @NullSafe
    String obj = object.thisMethodWillThrowIoException();
    return obj;
}

Here after AST manipulation by this plugin code will become

public String getSomeStuff(SomeClass object) {
    String obj = Optional.ofNullable(object)
                    .map(it -> it.thisMethodWillThrowIoException()) // In Java you can't have checked exception in lambda
                    .orElse(null);
    return obj;
}

Given Java Stream won't allow checked exception in lambda, you will get compilation error saying

java: unreported exception java.io.IOException; must be caught or declared to be thrown

Does not work with eclipse

Eclipse use its own compiler ECJ(Eclipse compiler for Java), and I could not find any way to pass in -Xplugin:ElvisPlugin. I am still trying to find way where I can patch this.