Allow override of StreamReadContraints default with `overrideDefaul…

…tStreamReadConstraints()` (#1019)

src/main/java/com/fasterxml/jackson/core/StreamReadConstraints.java

@@ -57,9 +57,34 @@ public class StreamReadConstraints
     protected final int _maxNumLen;
     protected final int _maxStringLen;
 
-    private static final StreamReadConstraints DEFAULT =
+    private static StreamReadConstraints DEFAULT =
         new StreamReadConstraints(DEFAULT_MAX_DEPTH, DEFAULT_MAX_NUM_LEN, DEFAULT_MAX_STRING_LEN);
 
+    /**
+     * Override the default StreamReadConstraints. These defaults are only used when {@link JsonFactory}
+     * instances are not configured with their own StreamReadConstraints.
+     * <p>
+     * Library maintainers should not set this as it will affect other code that uses Jackson.
+     * Library maintainers who want to configure StreamReadConstraints for the Jackson usage within their
+     * lib should create <code>ObjectMapper</code> instances that have a {@link JsonFactory} instance with
+     * the required StreamReadConstraints.
+     * <p>
+     * This method is meant for users delivering applications. If they use this, they set it when they start
+     * their application to avoid having other code initialize their mappers before the defaults are overridden.
+     *
+     * @param streamReadConstraints new default for StreamReadConstraints (a null value will reset to built-in default)
+     * @see #defaults()
+     * @see #builder()
+     * @since v2.15.2
+     */
+    public static void overrideDefaultStreamReadConstraints(final StreamReadConstraints streamReadConstraints) {
+        if (streamReadConstraints == null) {
+            DEFAULT = new StreamReadConstraints(DEFAULT_MAX_DEPTH, DEFAULT_MAX_NUM_LEN, DEFAULT_MAX_STRING_LEN);
+        } else {
+            DEFAULT = streamReadConstraints;
+        }
+    }
+
     public static final class Builder {
         private int maxNestingDepth;
         private int maxNumLen;
@@ -161,6 +186,10 @@ public static Builder builder() {
         return new Builder();
     }
 
+    /**
+     * @return the default {@link StreamReadConstraints} (when none is set on the {@link JsonFactory} explicitly)
+     * @see #overrideDefaultStreamReadConstraints
+     */
     public static StreamReadConstraints defaults() {
         return DEFAULT;
     }

src/test/java/com/fasterxml/jackson/core/TestStreamReadConstraints.java

@@ -0,0 +1,33 @@
+package com.fasterxml.jackson.core;
+
+import org.junit.Test;
+
+import static org.junit.Assert.assertEquals;
+
+public class TestStreamReadConstraints {
+    @Test
+    public void testOverride() {
+        final int numLen = 1234;
+        final int strLen = 12345;
+        final int depth = 123;
+        StreamReadConstraints constraints = StreamReadConstraints.builder()
+                .maxNumberLength(numLen)
+                .maxStringLength(strLen)
+                .maxNestingDepth(depth)
+                .build();
+        try {
+            StreamReadConstraints.overrideDefaultStreamReadConstraints(constraints);
+            assertEquals(depth, StreamReadConstraints.defaults().getMaxNestingDepth());
+            assertEquals(strLen, StreamReadConstraints.defaults().getMaxStringLength());
+            assertEquals(numLen, StreamReadConstraints.defaults().getMaxNumberLength());
+        } finally {
+            StreamReadConstraints.overrideDefaultStreamReadConstraints(null);
+            assertEquals(StreamReadConstraints.DEFAULT_MAX_DEPTH,
+                    StreamReadConstraints.defaults().getMaxNestingDepth());
+            assertEquals(StreamReadConstraints.DEFAULT_MAX_STRING_LEN,
+                    StreamReadConstraints.defaults().getMaxStringLength());
+            assertEquals(StreamReadConstraints.DEFAULT_MAX_NUM_LEN,
+                    StreamReadConstraints.defaults().getMaxNumberLength());
+        }
+    }
+}