8295537: Enhance TRACE_METHOD_LINKAGE to show the target MethodHandle

Reviewed-by: jvernee, redestad, mchung

src/java.base/share/classes/java/lang/invoke/BoundMethodHandle.java

@@ -158,21 +158,29 @@ abstract non-sealed class BoundMethodHandle extends MethodHandle {
     final int fieldCount() { return speciesData().fieldCount(); }
 
     @Override
-    Object internalProperties() {
-        return "\n& BMH="+internalValues();
+    Object internalProperties(int indentLevel) {
+        return "\n" + debugPrefix(indentLevel) + "& BMH=" + internalValues(indentLevel);
     }
 
     @Override
-    final String internalValues() {
+    final String internalValues(int indentLevel) {
+        String prefix = debugPrefix(indentLevel);
         int count = fieldCount();
-        if (count == 1) {
+        if (count == 1 && indentLevel < 0) {
             return "[" + arg(0) + "]";
         }
         StringBuilder sb = new StringBuilder("[");
         for (int i = 0; i < count; ++i) {
-            sb.append("\n  ").append(i).append(": ( ").append(arg(i)).append(" )");
+            Object theArg = arg(i);
+            sb.append("\n  ").append(prefix).append(i);
+            if (indentLevel >= 0 && theArg instanceof MethodHandle) {
+                sb.append(": MethodHandle = {").append(((MethodHandle)theArg).debugString(indentLevel+1));
+                sb.append("\n  ").append(prefix).append("}");
+            } else {
+                sb.append(": ( ").append(theArg).append(" )");
+            }
         }
-        return sb.append("\n]").toString();
+        return sb.append("\n").append(prefix).append("]").toString();
     }
 
     /*non-public*/

src/java.base/share/classes/java/lang/invoke/DirectMethodHandle.java

@@ -170,8 +170,8 @@ sealed class DirectMethodHandle extends MethodHandle {
     }
 
     @Override
-    String internalProperties() {
-        return "\n& DMH.MN="+internalMemberName();
+    String internalProperties(int indentLevel) {
+        return "\n" + debugPrefix(indentLevel) + "& DMH.MN=" + internalMemberName();
     }
 
     //// Implementation methods.

src/java.base/share/classes/java/lang/invoke/LambdaForm.java

@@ -1029,12 +1029,18 @@ class LambdaForm {
     }
 
     public String toString() {
+        return debugString(-1);
+    }
+
+    String debugString(int indentLevel) {
+        String prefix = MethodHandle.debugPrefix(indentLevel);
         String lambdaName = lambdaName();
-        StringBuilder buf = new StringBuilder(lambdaName + "=Lambda(");
+        StringBuilder buf = new StringBuilder(lambdaName);
+        buf.append("=Lambda(");
         for (int i = 0; i < names.length; i++) {
             if (i == arity)  buf.append(")=>{");
             Name n = names[i];
-            if (i >= arity)  buf.append("\n    ");
+            if (i >= arity)  buf.append("\n    ").append(prefix);
             buf.append(n.paramString());
             if (i < arity) {
                 if (i+1 < arity)  buf.append(",");

src/java.base/share/classes/java/lang/invoke/MethodHandle.java

@@ -1688,9 +1688,19 @@ assertEquals("[three, thee, tee]", asListFix.invoke((Object)argv).toString());
     }
     /** Return a string with a several lines describing the method handle structure.
      *  This string would be suitable for display in an IDE debugger.
+     * @param indentLevel If negative, return only information about this MethodHandle.
+     *  Otherwise, return information about this MethodHandle and (recursively) all other
+     *  MethodHandles, if any, that are invoked directly or indirectly by this MethodHandle.
+     *  During the recursion, `indentLevel` is incremented (see
+     *  BoundMethodHandle.internalValues()) to improve readability of
+     *  the nested structure.
      */
+    String debugString(int indentLevel) {
+        return type + " : " + internalForm().debugString(indentLevel) +
+               internalProperties(indentLevel);
+    }
     String debugString() {
-        return type+" : "+internalForm()+internalProperties();
+        return debugString(-1);
     }
 
     //// Implementation methods.
@@ -1787,12 +1797,29 @@ assertEquals("[three, thee, tee]", asListFix.invoke((Object)argv).toString());
     }
 
     /*non-public*/
-    Object internalValues() {
+    Object internalValues(int indentLevel) {
         return null;
     }
 
+    /**
+     * Various debugging methods in MethodHandle (and subclasses thereof) and LambdaForm
+     * take an `indentLevel` argument, so that {@link java.lang.invoke.MethodHandle.debugString(int)}
+     * can return nested structures in a readable fashion. This method returns a string to be
+     * prepended to each line at the specified level.
+     */
+    static String debugPrefix(int indentLevel) {
+        if (indentLevel <= 0) {
+            return "";
+        }
+        return "    ".repeat(indentLevel);
+    }
+
     /*non-public*/
     Object internalProperties() {
+        return internalProperties(-1);
+    }
+
+    Object internalProperties(int indentLevel) {
         // Override to something to follow this.form, like "\n& FOO=bar"
         return "";
     }

src/java.base/share/classes/java/lang/invoke/MethodHandleNatives.java

@@ -281,6 +281,12 @@ class MethodHandleNatives {
                                               type,
                                               staticArguments,
                                               caller);
+        if (TRACE_METHOD_LINKAGE) {
+            MethodHandle target = callSite.getTarget();
+            System.out.println("linkCallSite target class => " + target.getClass().getName());
+            System.out.println("linkCallSite target => " + target.debugString(0));
+        }
+
         if (callSite instanceof ConstantCallSite) {
             appendixResult[0] = callSite.dynamicInvoker();
             return Invokers.linkToTargetMethod(type);
@@ -298,13 +304,13 @@ class MethodHandleNatives {
         Object bsmReference = bootstrapMethod.internalMemberName();
         if (bsmReference == null)  bsmReference = bootstrapMethod;
         String staticArglist = staticArglistForTrace(staticArguments);
-        System.out.println("linkCallSite "+caller.getName()+" "+
+        System.out.println("linkCallSite "+getCallerInfo(caller)+" "+
                            bsmReference+" "+
                            name+type+"/"+staticArglist);
         try {
             MemberName res = linkCallSiteImpl(caller, bootstrapMethod, name, type,
                                               staticArguments, appendixResult);
-            System.out.println("linkCallSite => "+res+" + "+appendixResult[0]);
+            System.out.println("linkCallSite linkage => "+res+" + "+appendixResult[0]);
             return res;
         } catch (Throwable ex) {
             ex.printStackTrace(); // print now in case exception is swallowed
@@ -313,6 +319,20 @@ class MethodHandleNatives {
         }
     }
 
+    /**
+     * Return a human-readable description of the caller. Something like
+     * "java.base/java.security.Security.<clinit>(Security.java:82)"
+     */
+    private static String getCallerInfo(Class<?> caller) {
+        for (StackTraceElement e : Thread.currentThread().getStackTrace()) {
+            if (e.getClassName().equals(caller.getName())) {
+                return e.toString();
+            }
+        }
+        // fallback if the caller is somehow missing from the stack.
+        return caller.getName();
+    }
+
     // this implements the upcall from the JVM, MethodHandleNatives.linkDynamicConstant:
     static Object linkDynamicConstant(Object callerObj,
                                       Object bootstrapMethodObj,