## What changes were proposed in this pull request?
This patch tries to implement this [proposal](https://github.com/apache/spark/pull/19813#issuecomment-354045400) to add an API for handling expression code generation. It should allow us to manipulate how to generate codes for expressions.
In details, this adds an new abstraction `CodeBlock` to `JavaCode`. `CodeBlock` holds the code snippet and inputs for generating actual java code.
For example, in following java code:
```java
int ${variable} = 1;
boolean ${isNull} = ${CodeGenerator.defaultValue(BooleanType)};
```
`variable`, `isNull` are two `VariableValue` and `CodeGenerator.defaultValue(BooleanType)` is a string. They are all inputs to this code block and held by `CodeBlock` representing this code.
For codegen, we provide a specified string interpolator `code`, so you can define a code like this:
```scala
val codeBlock =
code"""
|int ${variable} = 1;
|boolean ${isNull} = ${CodeGenerator.defaultValue(BooleanType)};
""".stripMargin
// Generates actual java code.
codeBlock.toString
```
Because those inputs are held separately in `CodeBlock` before generating code, we can safely manipulate them, e.g., replacing statements to aliased variables, etc..
## How was this patch tested?
Added tests.
Author: Liang-Chi Hsieh <viirya@gmail.com>
Closes#21193 from viirya/SPARK-24121.
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