Add constant STATIC_INITIALIZER for allowBlockingCalls

This commit introduces a STATIC_INITIALIZER constant in BlockHound, so
it becomes easier to discover that the features relying on the
`String methodName` parameter can work with `<clinit>` for static
initializer.

Additionally, this is documented in the customization examples and
used in the StaticInitTest.

Fixes #178.

agent/src/main/java/reactor/blockhound/BlockHound.java

@@ -62,7 +62,13 @@
  */
 public class BlockHound {
 
-    static final String PREFIX = "$$BlockHound$$_";
+    /**
+     * The special methodName that should be used in e.g. {@link Builder#allowBlockingCallsInside(String, String)}
+     * when one wants to reference the static initializer block rather than a class method.
+     */
+    public static final String STATIC_INITIALIZER = "<clinit>";
+
+    static final String PREFIX                 = "$$BlockHound$$_";
 
     private static final AtomicBoolean INITIALIZED = new AtomicBoolean(false);
 
@@ -294,6 +300,7 @@ public Builder markAsBlocking(String className, String methodName, String signat
          * @param className class' name (e.g. "java.lang.Thread")
          * @param methodName a method name
          * @return this
+         * @see BlockHound#STATIC_INITIALIZER
          */
         public Builder allowBlockingCallsInside(String className, String methodName) {
             allowances.computeIfAbsent(className, __ -> new HashMap<>()).put(methodName, true);
@@ -307,6 +314,7 @@ public Builder allowBlockingCallsInside(String className, String methodName) {
          * @param className class' name (e.g. "java.lang.Thread")
          * @param methodName a method name
          * @return this
+         * @see BlockHound#STATIC_INITIALIZER
          */
         public Builder disallowBlockingCallsInside(String className, String methodName) {
             allowances.computeIfAbsent(className, __ -> new HashMap<>()).put(methodName, false);

docs/customization.md

@@ -40,6 +40,14 @@ builder.disallowBlockingCallsInside(
 );
 ```
 
+This will allow blocking method calls inside the static initializer block of `MyClass` down the callstack:
+```java
+builder.allowBlockingCallsInside(
+    "com.example.MyClass",
+    BlockHound.STATIC_INITIALIZER
+);
+```
+
 ## Custom blocking method callback
 * `Builder#blockingMethodCallback(Consumer<BlockingMethod> consumer)`
 

example/src/test/java/com/example/StaticInitTest.java

@@ -25,7 +25,7 @@ public class StaticInitTest {
 
     static {
         BlockHound.install(b -> {
-            b.allowBlockingCallsInside(ClassWithStaticInit.class.getName(), "<clinit>");
+            b.allowBlockingCallsInside(ClassWithStaticInit.class.getName(), BlockHound.STATIC_INITIALIZER);
         });
     }