Add set-default option

This option allows to install an additional JDK without making it the
default one.

I have wanted this for quite a long time as I'm running custom GitHub
Actions with Java, which might require a specific JDK and I don't want
to pollute the JDK that is used by the overall workflow calling the
action.
And I'm apparently not alone as there was a preexisting issue.

Fixes #560

src/distributions/base-installer.ts

@@ -20,6 +20,7 @@ export abstract class JavaBase {
   protected packageType: string;
   protected stable: boolean;
   protected checkLatest: boolean;
+  protected setDefault: boolean;
 
   constructor(
     protected distribution: string,
@@ -36,6 +37,10 @@ export abstract class JavaBase {
     this.architecture = installerOptions.architecture || os.arch();
     this.packageType = installerOptions.packageType;
     this.checkLatest = installerOptions.checkLatest;
+    this.setDefault =
+      installerOptions.setDefault !== undefined
+        ? installerOptions.setDefault
+        : true;
   }
 
   protected abstract downloadTool(
@@ -169,8 +174,15 @@ export abstract class JavaBase {
       foundJava.path = macOSPostfixPath;
     }
 
-    core.info(`Setting Java ${foundJava.version} as the default`);
-    this.setJavaDefault(foundJava.version, foundJava.path);
+    if (this.setDefault) {
+      core.info(`Setting Java ${foundJava.version} as the default`);
+      this.setJavaDefault(foundJava.version, foundJava.path);
+    } else {
+      core.info(
+        `Installing Java ${foundJava.version} (not setting as default)`
+      );
+      this.setJavaEnvironment(foundJava.version, foundJava.path);
+    }
 
     return foundJava;
   }
@@ -310,6 +322,17 @@ export abstract class JavaBase {
     );
   }
 
+  protected setJavaEnvironment(version: string, toolPath: string) {
+    const majorVersion = version.split('.')[0];
+    core.setOutput('distribution', this.distribution);
+    core.setOutput('path', toolPath);
+    core.setOutput('version', version);
+    core.exportVariable(
+      `JAVA_HOME_${majorVersion}_${this.architecture.toUpperCase()}`,
+      toolPath
+    );
+  }
+
   protected distributionArchitecture(): string {
     // default mappings of config architectures to distribution architectures
     // override if a distribution uses any different names; see liberica for an example