Compile tests using Java 17

[nedtwigg]

It would be really convenient if our tests could use multiline string literals. Then we could remove some of this cuteness

spotless/testlib/src/main/java/com/diffplug/spotless/ResourceHarness.java

Lines 184 to 186 in aa78b56

	public File toLines(String... lines) {
	return toContent(String.join("\n", Arrays.asList(lines)));
	}

spotless/plugin-gradle/src/test/java/com/diffplug/gradle/spotless/ConfigurationCacheTest.java

Lines 50 to 61 in aa78b56

	setFile("build.gradle").toLines(
	"plugins {",
	" id 'com.diffplug.spotless'",
	"}",
	"repositories { mavenCentral() }",
	"apply plugin: 'java'",
	"spotless {",
	" java {",
	" googleJavaFormat('1.2')",
	" }",
	"}",
	"tasks.named('spotlessJavaApply').get()");

Right now we test on two platforms - jdk11 and jdk17. But once #1304 gets figured out, we could run our build on Java 17, up the source level to 17 for tests, and use Gradle toolchain to run tests against jre 11 + 17.

That might be too complicated, and java 11 LTS reaches end of life in 2026 which will be here before we know it anyway. But if you made a PR that

• preserved the existing test matrix
• but always used java 17 as the build jre
• compiled bytecode to java 11 for the jars
• compiled bytecode to java 17 for the tests

That would be welcome and quickly merged!

[Goooler]

Use toolchains?

https://docs.gradle.org/current/userguide/toolchains.html
https://jakewharton.com/build-on-latest-java-test-through-lowest-java